Methods, systems, and computer-program products for estimating scattered radiation in radiographic projections

ABSTRACT

Several related inventions for estimating scattered radiation in radiographic projections are disclosed. Several of the inventions use scatter kernels of various forms, including symmetric and asymmetric forms. The inventions may be used alone or in various combinations with one another. The resulting estimates of scattered radiation may be used to correct the projections, which can improve the results of tomographic reconstructions. Still other inventions of the present application generate estimates of scattered radiation from shaded or partially shaded regions of a radiographic projection, which may be used to correct the projections or used to adjust the estimates of scattered radiation generated according to inventions of the present application that employ kernels.

This application is a Continuation of U.S. patent application Ser. No.12/125,053, filed May 21, 2008, now allowed, entitled “METHODS, SYSTEMS,AND COMPUTER-PROGRAM PRODUCTS FOR ESTIMATING SCATTERED RADIATION INRADIOGRAPHIC PROJECTIONS,” the content of which is incorporated hereinby reference in its entirety for any and all purposes.

BACKGROUND OF THE INVENTION

Computerized tomography (CT) involves the imaging of the internalstructure of an object by collecting several projection images(“radiographic projections”) in a single scan operation (“scan”), and iswidely used in the medical field to view the internal structure ofselected portions of the human body. Typically, several two-dimensionalprojections are made of the object, and a three-dimensionalrepresentation of the object is constructed from the projections usingvarious tomographic reconstruction methods. From the three-dimensionalimage, conventional CT slices through the object can be generated. Thetwo-dimensional projections are typically created by transmittingradiation from a “point source” through the object, which will absorbsome of the radiation based on its size, density, and atomiccomposition, and collecting the non-absorbed radiation onto atwo-dimensional imaging device, or imager, which comprises an array ofpixel detectors (simply called “pixels”). Such a system is shown inFIG. 1. Typically, the point source and the center of thetwo-dimensional imager lie on a common axis, which may be called theprojection axis. The source's radiation emanates toward the imagingdevice in a volume of space defined by a right-circular, elliptical, orrectangular cone having its vertex at the point source and its base atthe imaging device. For this reason, the radiation is often calledcone-beam (CB) radiation. Generally, when no object is present withinthe cone, the distribution of radiation is substantially uniform on anycircular area on the imager that is centered about the projection axis,and that is within the cone. However, the distribution of the radiationmay be slightly non-uniform, while having rotational symmetry about theprojection axis. In any event, any non-uniformity in the distributioncan be measured in a calibration step and accounted for. The projectionaxis may not be at the center of the imager or the center of the object.It may pass through them at arbitrary locations including very near theedge.

In an ideal imaging system, rays of radiation travel along respectivestraight-line transmission paths from the source, through the object,and then to respective pixel detectors without generating scatteredrays. However, in real systems, when a quantum of radiation is absorbedby a portion of the object, one or more scattered rays are oftengenerated that deviate from the transmission path of the incidentradiation. These scattered rays are often received by “surrounding”pixel detectors that are not located on the transmission path that theinitial quantum of radiation was transmitted on, thereby creating errorsin the electrical signals of the surrounding pixel detectors.Furthermore, in typical two-dimensional imagers, the radiation meant tobe received by a pixel is often distributed by various components of theimager (e.g., scintillation plate), and received by surrounding pixels.Also, there is typically some electrical cross-talk in the electricalsignals of the pixel detectors caused by the electrical circuitry thatreads the array of pixel detectors. These two effects are oftencharacterized by a point-spread function (PSF), which is atwo-dimensional mapping of the amount of error caused in surroundingpixels by a given amount of radiation received by a central pixel. Thesurface of the PSF is similar to the flared shape of a trumpet output,with the greatest amount of error occurring in pixels adjacent to thecentral pixel. Each of these non-ideal effects creates spatial errors inthe pixel data generated by the two-dimensional imager.

The scattered radiation causes artifacts (e.g., phantom images) and lossof resolution and contrast in the CT image slices produced by theradiation imaging system. The scattered radiation can also causenumerical errors in the image reconstruction algorithms (generallyreferred to as “CT number problems” in the art). All of the foregoinglead to image degradation. Accordingly, there is a need in thecomputerized tomography field to reduce the impacts of these spatial andtemporal errors.

BRIEF SUMMARY OF THE INVENTIONS

Several related inventions are disclosed by the present invention. Theinventions may be used alone or in various combinations with oneanother. As used herein and in the claims, the action of obtaining anitem, such as an estimate of a quantity, encompasses the action ofreceiving the item from an outside process, computer-program product,and/or system, and the action of generating the item by the claimedprocess, computer-program product, or system.

A first set of inventions relates to methods, computer-program products,and systems for estimating scattered radiation in a radiographicprojection of an object using a symmetric kernel. The radiographicprojection is generated by a two-dimensional imaging device irradiatedby a radiation source spaced therefrom to provide a space for theobject. The imaging device measures incident radiation at a plurality ofpixels at corresponding locations on a two-dimensional surface, and theradiographic projection comprises a two-dimensional surface and aplurality of radiation values corresponding to a plurality of pixellocations of the imaging device. Each radiation value includes a primaryradiation amount (e.g., component) representative of the radiationreaching the corresponding pixel along a direct path from the radiationsource and a scattered radiation amount (e.g., component) representativeof other radiation reaching the corresponding pixel.

An exemplary method embodiment according to the first set of inventionsbroadly comprises obtaining an estimate of a radiation amount associatedwith a first location of the radiographic projection, the radiationamount comprising one of a radiation amount emitted by the radiationsource or the primary radiation amount at the first location. Theexemplary method further comprises generating an estimate of scatteredradiation at a plurality of locations of the radiographic projectionusing the estimate of the radiation amount at the first location and akernel. The kernel generates a value representative of scatteredradiation for a location of the radiographic projection in relation tothe distance between that location and the first location. The kernelcomprises at least two symmetric functions, each symmetric functionhaving radial symmetry about the location of the first pixel location.The exemplary method further comprises storing the estimate of thescattered radiation in a computer-readable medium.

An exemplary computer-program product embodiment according to the firstset of inventions broadly comprises instruction sets embodied on acomputer-readable medium which, when executed by a processor of acomputer system, cause the processor to implement the actions of theexemplary method embodiment. An exemplary system embodiment according tothe first set of inventions broadly encompasses a radiation source, atwo-dimensional imaging device, a processor, and a set of instructionsstored on a computer-readable medium to implement actions that includethe actions of the above exemplary method embodiment according to thefirst set of inventions.

A second set of inventions relates to methods, computer-programproducts, and systems for estimating scattered radiation in aradiographic projection of an object using an asymmetric kernel. Theradiographic projection is generated by a two-dimensional imaging deviceirradiated by a radiation source spaced therefrom to provide a space forthe object. The imaging device measures incident radiation at aplurality of pixels at corresponding locations on a two-dimensionalsurface, and the radiographic projection comprises a two-dimensionalsurface and a plurality of radiation values corresponding to a pluralityof pixel locations of the imaging device. Each radiation value includesa primary radiation amount (e.g., component) representative of theradiation reaching the corresponding pixel along a direct path from theradiation source and a scattered radiation amount (e.g., component)representative of other radiation reaching the corresponding pixel.

An exemplary method embodiment according to the second set of inventionsbroadly comprises obtaining an estimate of a radiation amount associatedwith a first location of the radiographic projection, the radiationamount comprising one of a radiation amount emitted by the radiationsource or the primary radiation amount at the first location. Theexemplary method further comprises generating an estimate of scatteredradiation at a plurality of locations of the radiographic projectionusing the estimate of the radiation amount at the first location and akernel that generates a value representative of scattered radiation fora location of the radiographic projection in relation to the distancebetween that location and the first location, the kernel having a formthat is asymmetric about the location of the first pixel location. Theexemplary method further comprises storing the estimate of the scatteredradiation in a computer-readable medium.

An exemplary computer-program product embodiment according to the secondset of inventions broadly comprises instruction sets embodied on acomputer-readable medium which, when executed by a processor of acomputer system, cause the processor to implement the actions of theexemplary method embodiment. An exemplary system embodiment according tothe second set of inventions broadly encompasses a radiation source, atwo-dimensional imaging device, a processor, and a set of instructionsstored on a computer-readable medium to implement actions that includethe actions of the above exemplary method embodiment according to thesecond set of inventions.

A third set of inventions relates to methods, computer-program products,and systems for estimating scattered radiation in a radiographicprojection of an object using two or more kernels having variedcharacteristics. Each kernel may have a symmetric form or an asymmetricform. The radiographic projection is generated by a two-dimensionalimaging device irradiated by a radiation source spaced therefrom toprovide a space for the object. The imaging device measures incidentradiation at a plurality of pixels at corresponding locations on atwo-dimensional surface, and the radiographic projection comprises atwo-dimensional surface and a plurality of radiation valuescorresponding to a plurality of pixel locations of the imaging device.Each radiation value includes a primary radiation amount (e.g.,component) representative of the radiation reaching the correspondingpixel along a direct path from the radiation source and a scatteredradiation amount (e.g., component) representative of other radiationreaching the corresponding pixel.

An exemplary method embodiment according to the third set of inventionsbroadly comprises obtaining a plurality of estimates of a radiationamount associated at a corresponding plurality of locations of theradiographic projection, each radiation amount comprising one of aradiation amount emitted by the radiation source or the primaryradiation amount at the corresponding location. The exemplary methodfurther comprises generating an estimate of scattered radiation at aplurality of locations of the radiographic projection using an estimateof the radiation amount at a first location of the radiographicprojection with a first kernel that generates a value representative ofscattered radiation for a location of the radiographic projection inrelation to the distance between that location and the first location,and further using an estimate of the radiation amount at a secondlocation of the radiographic projection with a second kernel thatgenerates a value representative of scattered radiation for a locationof the radiographic projection in relation to the distance between thatlocation and the second location. Each kernel comprises a form thatrelates its value to the distance, wherein the first and second kernelshave different forms. The form of each kernel may be symmetric orasymmetric. The exemplary method further comprises storing the estimateof the scattered radiation in a computer-readable medium.

An exemplary computer-program product embodiment according to the thirdset of inventions broadly comprises instruction sets embodied on acomputer-readable medium which, when executed by a processor of acomputer system, cause the processor to implement the actions of theexemplary method embodiment. An exemplary system embodiment according tothe third set of inventions broadly encompasses a radiation source, atwo-dimensional imaging device, a processor, and a set of instructionsstored on a computer-readable medium to implement actions that includethe actions of the above exemplary method embodiment according to thethird set of inventions.

A fourth set of inventions relates to methods, computer-programproducts, and systems for estimating scattered radiation in at least oneshaded or partially shaded region of a radiographic projection. Theestimate may be used to correct the radiographic projection or used toadjust the estimates of scattered radiation generated according to otherinventions of the present application. The radiographic projection isgenerated by a two-dimensional imaging device irradiated by a radiationsource spaced therefrom to provide a space for the object. The imagingdevice measures incident radiation at a plurality of pixels atcorresponding locations on a two-dimensional surface, and theradiographic projection comprises a two-dimensional surface and aplurality of radiation values corresponding to a plurality of pixellocations of the imaging device. Each radiation value includes a primaryradiation amount (e.g., component) representative of the radiationreaching the corresponding pixel along a direct path from the radiationsource and a scattered radiation amount (e.g., component) representativeof other radiation reaching the corresponding pixel. The radiographicprojection has at least one region that has been at least partiallyshaded by a second object such that the region is irradiated by thepenumbra of the source.

An exemplary method embodiment according to the fourth set of inventionsbroadly comprises fitting a mathematical form to a plurality ofradiation values of the at least one region of the radiographicprojection that has been at least partially shaded by a second objectsuch that the region is irradiated by the penumbra of the source. Theexemplary method further comprises generating an estimate of scatteredradiation in the at least one region from the fitted mathematical form,and storing the estimate of the scattered radiation in acomputer-readable medium.

An exemplary computer-program product embodiment according to the fourthset of inventions broadly comprises instruction sets embodied on acomputer-readable medium which, when executed by a processor of acomputer system, cause the processor to implement the actions of theexemplary method embodiment. An exemplary system embodiment according tothe fourth set of inventions broadly encompasses a radiation source, atwo-dimensional imaging device, a processor, and a set of instructionsstored on a computer-readable medium to implement actions that includethe actions of the above exemplary method embodiment according to thefourth set of inventions.

A fifth set of inventions relates to methods, computer-program products,and systems for estimating scattered radiation in a radiographicprojection caused by the housing of an imaging device. These inventionsare more fully claimed in the attached claims and described in thedetailed description section.

The above inventions and other inventions of the present application,and additional embodiments thereof, are described in the DetailedDescription with reference to the Figures. In the Figures, like numeralsmay reference like elements and descriptions of some elements may not berepeated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a radiation imaging system according tothe prior art.

FIG. 2A is a schematic diagram of a first exemplary embodiment of aradiation imaging system according to some inventions of the presentapplication.

FIG. 2B is a schematic diagram of a second exemplary embodiment of aradiation imaging system according to some inventions of the presentapplication.

FIG. 3 is a perspective view of an exemplary implementation of a fanblade and a fan-blade drive according to some inventions of the presentapplication.

FIGS. 4A and 4B illustrate exemplary imaging chains according to someinventions of the present application.

FIG. 5 illustrates the construction of an exemplary detector housing.

FIG. 6 illustrates a two-dimensional distribution of scattered photonsresulting from a pencil beam striking the exemplary detector housingshown in FIG. 5 according to some inventions of the present application.

FIG. 7 illustrates a one-dimensional cross section of the distributionshown in FIG. 6 according to some inventions of the present application.

FIGS. 8A and 8B illustrate Monte Carlo simulations of a uniformrectangular slab having a thickness over a first range and the fit of afirst kernel instance to the simulated results according to someinventions of the present application.

FIGS. 9A and 9B illustrate Monte Carlo simulations of a uniformrectangular slab having a thickness over a second range and the fit of asecond kernel instance to the simulated results according to someinventions of the present application.

FIGS. 10A and 10B illustrate Monte Carlo simulations of a uniformrectangular slab having a thickness over a third range and the fit of athird kernel instance to the simulated results according to someinventions of the present application.

FIG. 11 illustrates the skewing effects on the scattered radiation,which are corrected according to some inventions of the presentapplication.

FIG. 12 is a top plan representation of the pixel array during aniteration where the pixels are divided into groups according to objectthickness, according to some inventions of the present application.

FIG. 13 shows a conventional Monte Carlo-based experiment of directing apencil beam of radiation at a perpendicular angle through a materialslab having a uniform thickness.

FIG. 14 shows an innovative Monte Carlo-based experiment by theinventors of directing a radiation pencil beam through a material slabhaving a wedge shape.

FIG. 15 illustrates the scatter distributions from objects havinguniform and non-uniform thicknesses according to several inventions ofthe present application.

FIG. 16 illustrates innovative Monte Carlo simulations of scatterradiation with the incident radiation beam located at varying distancesfrom an edge of a slab of material having uniform thickness according toseveral inventions of the present application.

FIG. 17 illustrates the change in kernel amplitude as a function of thedistance of the incident beam from the edge of a slab of material havinguniform thickness according to several inventions of the presentapplication.

FIG. 18 illustrates a configuration where the fan blades of the systemare positioned to provide shaded bands around all sides of an imagingdevice according to several inventions of the present application.

FIG. 19 illustrates a first configuration where the fan blades of thesystem are positioned to provide shaded bands around all sides of animaging device according to several inventions of the presentapplication.

FIG. 20 illustrates a second configuration where the fan blades of thesystem are positioned to provide shaded bands around all sides of animaging device according to several inventions of the presentapplication.

FIG. 21 illustrates the ideal radiation profile of a shaded regionwithout an object present according to several inventions of the presentapplication.

FIG. 22 illustrates a radiation profile of a shaded region with anobject present and with a fitting line made in the penumbra regionaccording to several inventions of the present application.

FIGS. 23, 24 and 25A-25D illustrates various results of applying some ofthe inventions of the present application.

DETAILED DESCRIPTION OF THE INVENTION

System Overview.

FIG. 2A is a schematic diagram of a first exemplary imaging system 100according to the system inventions of the present application. System100 comprises a radiation source 110, a two-dimensional imaging device120 disposed opposite to radiation source 110 along a projection line, afirst set of fan blades 130 disposed between the radiation source andthe two-dimensional imaging device, a first fan-blade drive 135 thatholds fan blades 130 and sets their positions. The edges of fan blades130 are oriented substantially perpendicular to the scan axis (definedbelow), and are substantially parallel with the trans-axial dimension(defined below) of imaging device 120. As an option, system 100 mayfurther comprise a second set of fan blades 140 disposed between theradiation source and the two-dimensional imaging device, and a secondfan-blade drive 145 that holds fan blades 140 and sets their positions.The edges of fan blades 140 are oriented substantially parallel with thescan axis (defined below), and are substantially perpendicular to theaxial dimension (defined below) of imaging device 120. The fan bladesare disposed closer to radiation source 110 than imaging device 120.They are normally kept wide open to enable the full extent of imagingdevice 120 to be exposed to radiation, but may be partially closedaccording to the image anchoring embodiments described near the end ofthis document. The dimensions of source 110, imaging device 120, and fanblades 130 and 140 (and their drives) have been enlarged in the figurerelative to the size of object for the purposes of illustrating thesecomponents.

System 100 further comprises a gantry 150 that holds radiation source110, imaging device 120, and fan-blade drives 135 and 145 in fixed orknown spatial relationships to one another, a mechanical drive 155 thatrotates gantry 150 about an object disposed between radiation source 110and imaging device 120, with the object being disposed between fanblades 130 and 140 on the one hand, and imaging device 120 on the otherhand. The term gantry has a broad meaning, and covers all configurationsof one or more structural members that can hold the above-identifiedcomponents in fixed or known (but possibly movable) spatialrelationships. For the sake of visual simplicity in the figure, thegantry housing, gantry support, and fan-blade support are not shown.These components do not form part of the present inventions. Also notshown are: a “bow tie” filter for radiation source 110, and a supporttable for the object (i.e., an object support member), neither of whichform a part of the present inventions related to system 100.Additionally, system 100 further comprises a controller 160 and a userinterface 165, with controller 160 being electrically coupled toradiation source 110, mechanical drive 155, fan-blade drives 135 and145, imaging device 120, and user interface 165. User interface 165provides a human interface to controller 160 that enables the user to atleast initiate a scan of the object, and to collect measured projectiondata from the imaging device. User interface 165 may be configured topresent graphic representations of the measured data.

In imaging system 100, gantry 150 is rotated about the object during ascan such that radiation source 110, fan blades 130 and 140, fan-bladedrives 135 and 145, and two-dimensional imaging device 120 circle aroundthe object. More specifically, gantry 150 rotates these components abouta scan axis, as shown in the figure, where the scan axis intersects theprojection line, and is typically perpendicular to the projection line.The object is aligned in a substantially fixed relationship to the scanaxis. The construction provides a relative rotation between theprojection line on the one hand and the scan axis and an object alignedthereto on the other hand, with the relative rotation being measured byan angular displacement value θ. Mechanical drive 155 is mechanicallycoupled to gantry 150 to provide rotation upon command by controller160. The two-dimensional imaging device comprises a two-dimensionalarray of pixels that are periodically read to obtain the data of theradiographic projections. Imaging device 120 has an X-axis and a Y-axis,which are perpendicular to each other. Imaging device 120 is orientedsuch that its Y-axis is parallel to the scan axis. For this reason, theY-axis is also referred to as the axial dimension of imaging device 120,and the X-axis is referred to as the trans-axial dimension, or lateraldimension, of device 120. The X-axis is perpendicular to a plane definedby the scan axis and the projection line, and the Y-axis is parallel tothis same plane. Each pixel is assigned a discrete X-coordinate (“X”)along the X-axis, and a discrete Y-coordinate (“Y”) along the Y-axis. Intypical implementations, the size of the array is 1024 pixels by 768pixels, measuring approximately 40 cm by 30 cm, with the longerdimension of the array being oriented parallel to the X-axis. As usedherein, the discrete X-coordinates start at −Xpix/2 and end at Xpix/2(e.g., Xpix=1024), and the discrete Y-coordinates start at −Ypix/2 andend at Ypix/2 (e.g., Ypix=768). A smaller number of pixels are shown inthe figure for the sake of visual clarity. The imaging device may becentered on the projection line to enable full-fan imaging of theobject, may be offset from the projection line to enable half-fanimaging of the object, or may be movable with respect to the projectionline to allow both full-fan and half-fan imaging of objects. As anexample of a half-fan configuration, the imaging device may be offsetfrom the center by 16 centimeters in its X-dimension when the imagingdevice has a span in the X dimension of 40 centimeters.

FIG. 3 shows a perspective view of a first exemplary implementation offan blades 130 and fan-blade drive 135. Each fan blade 130 may have athin rectangular shape, and may comprise a material that absorbs theradiation of source 110. Such a material may comprise lead (Pb). Eachfan blade 130 absorbs at least 60% of the incident radiation fromradiation source 110. In preferred implementations, a fan blade absorbsat least 90 percent, and more preferably at least 99 percent, of theradiation incident upon it. Fan-blade drive 135 may comprise twomechanical positioners. In one exemplary implementation, each mechanicalpositioner is mechanically coupled to a respective fan blade to causethe fan blade to move in a controlled and measurable (e.g., predictable)manner. In another implementation, one of the mechanical positioners ismechanically coupled to the fan blades to cause the blades to moverelative to one another so as to vary the distance of the gap betweenthe blades in a controlled and measurable manner, and the otherpositioner is mechanically coupled to the blades to cause the blades tomove as a group in a controlled and measurable manner. In the latterexemplary implementation, the first positioner and the fan blades may bemechanically disposed on a carriage, and the second positioner may bemechanically coupled to the carriage. Each positioner may comprise alinear motor servo, a rotating motor servo with a rotation-to-lineartranslation mechanism, or the like. The construction of fan blades 140and fan blade drive 145 may be the same as that of fan blades 130 andfan-blade drive 135, respectively.

Referring back to FIG. 2A, when controller 160 receives a request fromthe user to begin a scan of an object, controller 160 instructsfan-blade drives 135 and 145 to set the fan blades 130 and 140,respectively, in open positions (or in partially closed positions asdescribed in greater detail below for some embodiments), instructsmechanical drive 155 to begin a scan rotation of gantry 150, andinstructs radiation source 110 to begin emitting radiation. As itrotates, mechanical drive 155 provides controller 160 with an indicationof the angular displacement value θ. Controller 160 uses thisinformation to read the values of imaging device 120's pixel detectorsat selected angular displacement values θ to obtain the data for theradiographic projections. Typically, there are between 250 and 1000projections taken in the 360-degree scan rotation, with each projectionbeing spaced from adjacent projections by a set increment Δθ of angulardisplacement. The controller stores the data from each projection in amemory storage device, along with the angular displacement value θ atwhich the projection was taken. The scatter correction methods disclosedby the present application may be used to correct each projection forscattered radiation caused by scattering in the object and the top coverof imaging device 120.

Controller 160 comprises a processor, an instruction memory for storinginstruction sets that direct the operation of the processor, a datamemory that stores pixel and other data values used by the presentinventions implemented by the imaging system, and an I/O port managerthat provides input/output data exchange between processor 160 and eachof radiation source 110, mechanical drive 155, fan-blade drives 135 and145, and imaging device 120. The instruction memory and data memory arecoupled to the main processor through a first bidirectional bus, and maybe implemented as different sections of the same memory device. Becauseof the large amount of data provided by the two-dimensional imagingdevice, the I/O port manager is preferably coupled to the main processorthrough a second bidirectional bus. However, the I/O port manager may becoupled to the main processor by way of the first bidirectional bus. Theoperation of the processor is guided by a group of instruction setsstored in the instruction memory, which is an exemplary form ofcomputer-readable medium. Some of these instruction sets may direct theprocessor to generate estimates of scattered radiation in one or more ofthe projections. Additional instruction sets may direct the processor tocorrect the projections according to scatter-correction methodsdisclosed by the present application, or may direct the processor tostore the raw projection data to a computer-readable medium, from whichit may be exported to another processor that performs the correction.Some exemplary instruction sets are described below in greater detail.

In exemplary imaging system 100 shown in FIG. 2A, the gantry rotatesabout the object, which means that the projection line rotates about theobject and the scan axis. Instead, it may be appreciated that the objectand the scan axis may be rotated while the gantry and the projectionline are stationary. A second exemplary imaging system which rotates theobject is shown at 100′ in FIG. 2B. System 100′ comprises all of thecomponents of system 100, with the components being coupled to oneanother in the same way, except that the mechanical drive is coupled toan object support member, which holds the object being scanned. Insystem 100′, the gantry remains stationary while the mechanical driverotates the object support member and the object. System 100′ issuitable for industrial uses (e.g., scanning non-human objects), whereassystem 100 is suitable for medical uses (e.g., scanning human objects).

Sources of Radiation Scattering.

In typical cone-beam systems, there are generally four possiblecomponents that can scatter the radiation: the bow-tie filter (ifpresent), the object being scanned, an anti-scatter grid (if present),and the detector housing. FIG. 4A illustrates the sources of scatterthrough what we may call the imaging chain. Starting at the left, thesource radiation has an initial intensity profile I₀ before reaching thebow-tie filter. After passing through the bow-tie filter, a firstportion of I₀ is scattered in various directions as profile I_(bs),while a second portion of the radiation continues toward the object as aprofile I_(b0), but at an attenuated level compared to profile I₀. Thespatial distribution of the scattered radiation profile I_(bs) can besimulated by various computer-based methods, and confirmed bymeasurements (without the object in place, and/or with a uniform slab ofabsorbent material in the location of the object). Several embodimentsof inventions of the present application may be used to generateestimates of the I_(bs) profile.

After I_(b0) passes through the object, a first portion of it isscattered in various directions as a scatter profile I_(os), while asecond portion of it continues on toward the cover of imaging device 120as profile I_(o0), but at an attenuated level. Several embodiments ofinventions of the present application are used to generate estimates ofthe I_(os) profile, as described below in greater detail. The scatteredradiation profile I_(b0) also passes through the object, with a portionbeing absorbed, another portion being re-scattered, and the remainingportion being unaffected. The resulting scattered radiation from thebow-tie filter, as it exits the object, may be represented by theprofile I′_(bs). Profile I′_(bs) may be estimated by variouscomputer-based models given the I_(bs) profile and an approximatephantom model of the object (e.g., a body of uniform material havingapproximately the same shape as the object, and having scatter andabsorption characteristics near the average characteristics of theobject). The scattered radiation profiles I′_(bs) and I_(os) areadditive.

To reduce the scattered radiation before it reaches imaging device 120,a scatter-absorbing grid (so-called “anti-scatter” gird) may be disposedbetween the object and imaging device 120. The grid generally comprisesan array of elongated passageways having generally rectangular crosssections, with wider openings at the ends facing imaging device 120. Thepassageways are arranged to allow unobstructed paths for radiation thatfollow the straight-line paths from the source 110 to imaging device120, and to absorb substantial amounts of the scattered radiation, whichdoes not follow the straight-line paths. The effect of the grid is tosubstantially pass the non-scattered radiation I_(o0) from the object toimaging device 120, while absorbing substantial portions of the scatterradiation I′_(bs) and I_(os). As an advantage of the grid, the resultingscattered radiation that reaches the cover of imaging device 120 issubstantially reduced; it may be represented by I″_(bs) and I′_(os). Asa disadvantage, the grid generally obscures portions of the imagingdevice 120 due to the finite wall thickness of the grid's passageways.

After the object radiation profile I_(o0) passes through the detectorhousing, a first portion of it is scattered in various directions asprofile I_(ds), while a second portion of it continues to the pixeldetectors of imaging device 120, but at an attenuated level, as profileI_(d0). The scattered radiation profiles I″_(bs) and I′_(os) alsoencounter some further scattering by the cover, resulting in scatterprofiles I′″_(bs) and I″_(os). Several embodiments of inventions of thepresent application are used to estimate the scattering effects of thecover so that it can be removed from the measurement. The scatteringfrom a detector housing can sometimes be negligible, and can thus beignored.

Several embodiments of the present invention are directed to correctingfor the scattering caused by the object and the detector housing. Forthese embodiments, the simpler imaging chain shown in FIG. 4B may beused, which does not have the bow-tie filter or the anti-scatter grid.There are many system configurations which do not use bow-tie filtersand scatter grids. An initial radiation profile I₀ is directed towardthe object. After I₀ passes through the object, a first portion I_(os)of it is scattered in various directions, while a second portion I_(o0)of it continues on toward the cover of imaging 120, but at an attenuatedlevel. After the object radiation profile I_(o0) passes through thedetector housing, a first portion I_(ds) of it is scattered in variousdirections, while a second portion I_(d0) of it continues to the pixeldetectors of imaging 120, but at an attenuated level. The scatteredradiation profile I_(os) also encounters some further scattering by thecover, resulting in scatter profile I′_(os).

Correction of Scatter from the Detector Housing.

If scattering from the detector housing is not considered to benegligible, it may be corrected according to the following embodimentsof the present inventions. FIG. 5 illustrates the construction of anexemplary detector housing that comprises a outer shell, followed by athin electro-magnetic interference shield, followed by a low-densityspacer layer, followed by a thin scintillator layer (which converts theradiation into visible light), followed by a glass layer that comprisesthe pixel photo-detectors (which convert the visible light intoelectrical signals), followed by a support substrate. Most of thescattering occurs in the outer shell, with little scattering in the EMIshield (because of its small thickness) or the spacer layer (because ofits low density). The distance between the top of the outer shell andthe scintillator layer is 15.6 mm. The embodiments related to correctingthe scatter from the detector housing seek to relate the incidentradiation profile to the radiation profile measured by imaging device120, which can be represented by the profile Im. As it turns out, thescatter profiles for the imaging chains shown in FIGS. 4A and 4B,(I″_(bs)+I′_(os)) and I_(os), respectively, are relatively slowlyvarying over the spatial domain compared to main radiation profileI_(o0) from the object. This fact, in combination with the shortdistance of 15.6 mm between the top of the outer shell and thescintillator layer, allows the radiation incident on the detectorhousing to be modeled by a single incident profile I_(I) for thepurposes of estimating the scattering caused by the detector housing,and allows an accurate estimate of the incident profile I_(I) to begenerated from the measured radiation profile Im. Accordingly, theincident profile I_(I) is equal to (I_(o0)+I″_(bs)+I′_(os)) for theimaging chain shown in FIG. 4A, and equal to (I_(o0)+I_(os)) for theimaging chain shown in FIG. 4B.

Our next task is to find a relationship between profiles Im and I_(I)that will allow an estimate of profile I_(I) to be generated fromprofile Im. For each pixel on the imaging device, we can allocate apencil beam of radiation having incident radiation I_(I), and a primarybeam value Ip. The primary beam value Ip is the value of the pencil beamthat reaches the pixel after being attenuated and scattered by thedetector housing. The measured value Im at the pixel location comprisesthe primary value Ip and the scatter from other pencil beams. For adetector housing having uniform construction, Ip is a constant fractionof I_(I), and can be related as Ip(x,y)=η·I_(I)(x,y), for η slightlyless than 1, such as η=0.9. A computer-based Monte-Carlo simulation of apencil-beam of radiation interacting with the detector housing at aright angle was performed. The two-dimensional distribution of thescattered photons from the pencil beam is shown in FIG. 6, where thevertical axis plots the distribution on a logarithmic scale. FIG. 7shows a one-dimensional cross section of distribution, from the centeroutward as dots, where the vertical axis plots the distribution on alinear scale. The distribution can be mathematically modeled by apoint-scatter function PScF(r) as a function of distance r from thecenter of the pencil beam:PScF(r)=a ₁ ·e ^(−a) ² ^(r) +a ₃ ·e ^(−a) ⁴ ^((r−a) ⁵ ⁾ ³   [1]where a₁ through a₅ are fitting parameters. The fitting result is shownin FIG. 7 as a solid line. The PScF(r) function, as shown in FIGS. 6 and7, is not normalized to the cross sectional area of the pencil beam. Theparameters a₁ and a₃ may be adjusted so as to normalize PScF(r), orPScF(r) may be multiplied by a normalization constant. In any event, thepoint-scatter function should be multiplied by a calibration parameterto bring the simulation results into accordance with the measuredresults. The calibration parameter, which may be represented by thequantity SPR (scatter-to-primary ratio), is the ratio of the integral ofthe scattered-radiation component to the primary radiation.Computer-based Monte Carlo simulations suggest an SPR value of 9% forthe detector construction shown in FIG. 5, meaning that 9% of the beam'sradiation photons are scattered. This amount has been verified in partfrom shadowing experiments using opaque disks. While PScF(r) has beenillustrated as a single-value function of distance r, it may be writtenas a function of the x−y domain, PScF(x,y), where x and y are measuredfrom the center of the pencil beam, and where r=(x²+y²)^(1/2).

As indicated above, the incident radiation I_(I) can be modeled as anarray of pencil beams, with the measured value Im at the pixel locationcomprising the primary value Ip and the scatter from other pencil beams.That is, the radiation received by the imaging device 120 at any point(x,y), which is Im(x,y), is equal to the sum of Ip(x,y) and the scattercontributions from the other pencil beams. This may be writtenmathematically as:

$\begin{matrix}{{{Im}\left( {x,y} \right)} = {{{Ip}\left( {x,y} \right)} + {\sum\limits_{m = {- M}}^{+ M}{\sum\limits_{n = {- N}}^{+ N}{\left\lbrack {{{Ip}\left( {x_{m},y_{n}} \right)} \cdot {SPR} \cdot {{PScF}\left( {{x - x_{m}},{y - y_{n}}} \right)}} \right\rbrack.}}}}} & \lbrack 2\rbrack\end{matrix}$The latter quantity can be identified as the two-dimensional convolutionof Ip and PScF multiplied by SPR, which can be denoted as SPR·Ip(x,y)

PScF(x,y). Using this, we can write:Im(x,y)=Ip(x,y)+SPR·Ip(x,y)

PScF(x,y).  [3]As is known in the mathematics art, the Fourier transform of twoconvolved functions is equal to the multiplication of their Fouriertransforms. Thus, taking the Fourier transform of each side of equation[3], we have:ℑ{Im(x,y)}=ℑ{Ip(x,y)}+SPR·ℑ{Ip(x,y)}·ℑ{PScF(x,y)}which may be rewritten as:

$\begin{matrix}{{{??}\left\{ {{Ip}\left( {x,y} \right)} \right\}} = {{??}^{- 1}{\left\{ \frac{{??}\left( {I_{m}\left( {x,y} \right)} \right)}{1 + {{SPR} \cdot {{??}\left( {{PScF}\left( {x,y} \right)} \right)}}} \right\}.}}} & \lbrack 4\rbrack\end{matrix}$The inverse Fourier transform of equation [4] may be taken to obtainI_(I)(x,y) as follows:

$\begin{matrix}{{{Ip}\left( {x,y} \right)} = {{??}^{- 1}{\left\{ \frac{{??}\left( {I_{m}\left( {x,y} \right)} \right)}{1 + {{SPR} \cdot {{??}\left( {{PScF}\left( {x,y} \right)} \right)}}} \right\}.}}} & \lbrack 5\rbrack\end{matrix}$The denominator can be readily pre-computed, and Ip(x,y) can begenerated by a Fourier transform of Im(x,y), followed by a divisionoperation with the pre-computed denominator, and then followed by aninverse Fourier transform of the resulting division operation. This isgenerally faster than computing the convolution. I_(I)(x,y) may then begenerated as I_(I)(x,y)=Ip(x,y)/η. But since the η factor also exists inthe air scan (I₀) and will be cancelled out during reconstruction, anddoes not affect object scatter estimation, we can virtually neglect itand its actual value.

A simpler approach can be taken by approximating Ip(x,y) in theconvolution produce in equation [3] with Im(x,y) (or a fraction thereof)as follows:Im(x,y)=Ip(x,y)+SPR·Im(x,y)

PScF(x,y),  [6]which can be rewritten as:Ip(x,y)=[Im(x,y)−SPR·Im(x,y)

PScF(x,y)].  [7]I_(I)(x,y) may then be generated as I_(I)(x,y)=Ip(x,y)/η. Theconvolution product may be generated by multiplying the Fourier transferof Im(x,y) with a precomputed Fourier transform of PScF(x,y), andthereafter taking the inverse Fourier transform of the multiplicationproduct. As a practical matter, the calibration parameter SPR may bespatially variant. The variation may be caused by non-uniformities inthe thicknesses of the housing's topmost layers, and/or variations inthe thickness from the housing's top surface to the scintillator layer.In such a case, SPR can be modeled as a matrix, rather than a scalar,and the multiplication with the convolution in equation [7] becomes aninner product.

In typical implementations, an estimate of the scatter component{SPR·Ip(x,y)

PScF(x,y)} is generated on coarse grid of super pixels, where each superpixel represents a small group of pixels values and where SPR (in thecase of it being a matrix) and Ip(x,y) are each generated as an averageof values from its group. Ip(x,y) may be estimated as Im(x,y), afraction thereof, or estimated by either of equations [5] or [7]. Afterthe scatter component is generated on the coarse grid of super pixels,it may be interpolated/extrapolated to provide scatter values for all ofthe real pixels. A corrected radiographic projection may then begenerated as a difference between Im(x,y) and the scatter component,divided by η, or as a truncated difference between Im(x,y) and thescatter component, divided by η. The truncated difference may comprisegenerating the ratio of the scatter component to Im(x,y), processing theratio by filtering its values over the spatial domain and/or limitingits values of the ratio to a range (such 0 to 0.8), and then multiplyingIm(x,y) by one minus the processed ratio. The use of super pixels andgenerating average values for use therewith are described below ingreater detail. The full grid may also be used in generating the scattercomponent and Ip(x,y).

In the above ways, the radiation profile I_(I) incident on the detectorhousing can be generated from the measured radiation profile Im. Fromthere, the I_(I) can be used in the estimation of I″_(o0), I″_(bs), andI′_(os) in the imaging chain shown in FIG. 4A, or in the estimation ofI_(o0) and I_(os) in the imaging chain shown in FIG. 4B.

Radiation Scattering in the Object.

Scattered radiation profiles from the object are material- andgeometry-dependent, and, hence, are significantly more complicated thanthe scattered radiation profiles generated from the detector housingdescribed above. Prior art methods have assumed symmetric point-scatterfunctions for the object, with the functions being based on responsesthrough uniform slabs. One set of inventions of the present applicationrelates to a new form of a symmetric point-scatter function thatprovides improved projection-correction results. Another set ofinventions of the present application relates to constructing two ormore instances of the new symmetric form for two or more respectiveranges of object thickness, and applying the instances of the newsymmetric form to regions of a radiographic projection according to ameasure of the object thickness in the regions. Yet another set ofinventions of the present application relates to a new class ofasymmetric point-scatter functions that provide improvedprojection-correction results relative to symmetric point-scatterfunctions. Yet another set of inventions of the present applicationrelates to constructing two or more instances of the new asymmetric formfor two or more respective ranges of object thickness, and applying theinstances of the new asymmetric form to regions of a radiographicprojection according to a measure of the object thickness in theregions. Each of these invention sets is discussed below in greaterdetail, starting with the new form of a symmetric point-scatterfunction.

Symmetric Point-Scatter Functions.

In contrast to prior art approaches, one invention set of the presentapplication encompasses symmetric point-scatter functions (“kernels”)that comprise the sum of two or more symmetric functions. In preferredembodiments, the central portion of the scatter distribution (near thepencil-beam axis) is primarily modeled by one of the functions, whilethe tail portions are primarily modeled by the other symmetricfunction(s). In one embodiment, each function comprises a Gaussianfunction, and the kernel comprises the form:

$\begin{matrix}{{{{Sk}\left( {{x - x_{m}},{y - y_{n}},I_{0},I_{bp}} \right)} = {A \cdot \left( \frac{I_{bp}}{I_{0}} \right)^{\alpha} \cdot {\ln\left( \frac{I_{0}}{I_{bp}} \right)}^{\beta} \cdot \left( {{\exp\left( \frac{- r^{2}}{2\;\sigma_{1}^{2}} \right)} + {B\;{\exp\left( \frac{- r^{2}}{2\;\sigma_{2}^{2}} \right)}}} \right)}},} & \lbrack 8\rbrack\end{matrix}$where x and y are the location coordinates on the pixel array of thescattered radiation modeled by Sk(*), x_(m) and y_(n) are thecoordinates of the location where the pencil beam terminates on thepixel array, I₀ is the initial radiation intensity of the pencil beambefore striking the object, I_(bp) is the scatter-free radiationintensity of the pencil beam after passing through the object and asmeasured at the coordinates x_(m) and y_(n) on the pixel array, where A,B, α, β, σ₁, and σ₂ are the fitting parameters, and wherer²=(x−x_(m))²+(y−y_(n))² (r is the radial distance from point (x,y) topoint (x_(m), y_(n))). I_(bp) does not have scattered radiation in it,and is called the primary radiation of the object, or beam primarysignal, because it represents information about the region of the objecttraversed by the pencil-beam, and because its value is used by thetomographic reconstruction process to construct 3-D and images of theobject. Constant A and the first two factors of equation [8] primarilyaccount for the overall amount of scattering along the pencil beam andfor beam hardening effects. They also normalize kernel Sk(*) to I_(bp).The last term of equation [8] primarily accounts for the spatialdistribution of the scatter. It comprises two Gaussian functions, bothbeing functions of r²=(x−x_(m))²+(y−y_(n))².

The kernel form of equation [8] is normalized to I_(bp). It provides theratio of scattered radiation received at point (x,y) to the primaryradiation I_(bp) received at point (x_(m),y_(n)), and is dimensionless.The actual scattered radiation caused by the pencil beam is equal to thequantity I_(bp)·Sk(*). The kernel form of equation [8] may instead benormalized to the incident radiation I₀ by multiplying it with thequantity I_(bp)/I₀, which may be easily done by adding 1 to the value ofα. This normalized form may be called Sk^(Io)(*), and the actualscattered radiation caused by the pencil beam would be equal to thequantity I₀·Sk^(Io)(*). Sk^(Io)(*) has the same form of equation [8]except for a different value for α. Inventions of the presentapplication may be practiced with each of normalized kernel forms. Inaddition, each of the normalized kernels Sk^(Io)(*) and Sk(*) maycomprise one or more symmetric functions, each of which may comprise aGaussian function and/or other function.

Parameters A, B, σ₁, and σ₂ may be chosen so that the central portion ofthe scatter distribution is primarily modeled by one of the Gaussianfunctions, while the tail portions are primarily modeled by the otherGaussian function. For exemplary uses of Sk(*), it will be convenient togroup the first two terms and the constant A together as an amplitudefunction S_(A)(I₀, I_(bp)), which is a function of I₀ and I_(bp), and torepresent the third term as a form factor S_(FF)(x−x_(m), y−y_(n)),which is only a function of the coordinates x,y and the beam coordinatesx_(m),y_(n):Sk(x−x _(m) ,y−y _(n) ,I ₀ ,I _(bp))=S _(A)(I ₀ ,I _(bp))·S _(FF)(x−x_(m) ,y−y _(n))  [8A]For this form, both I₀ and I_(bp) are functions of x_(m) and y_(n),which means that S_(A)(I₀, I_(bp)) is a function of x_(m) and y_(n) forthis form.

FIG. 8A shows three Monte Carlo simulations of the normalized scatterdistributions (normalized to I_(bp)) from a perpendicular pencil beam(x_(m)=0, y_(n)=0) through three uniform rectangular slabs ofwater-based material (density of 1 g/cc) at three different thicknesses:3 cm, 5 cm, and 8 cm. The distributions are normalized to I_(bp), areplotted as a function of the radius r=[(x−x_(m))²+(y−y_(n))²]^(1/2), andare shown by dots in the figure. The value of I₀ for these simulationswas set to a typical value found in cone-beam CT systems. FIG. 8A alsoshows a first form Sk₁(*) of scatter kernel Sk(*) according to equation[8] fitted to the scattered distributions (solid lines) by using asuitable set of parameters A₁, B₁, α₁, β₁, σ_(1, 1), and σ_(2, 1). Oneexemplary set of parameters for r, x_(m), and y_(n) in units ofcentimeters is: A₁=9.47×10⁻⁵, B₁=2.2, α₁=−0.131, β₁=1.024, σ_(1,1)=19.5,and σ_(2,1)=3.23. The fit is excellent over the thickness range of 0 to10 cm. FIG. 8B shows the actual scatter distribution for the three slabthicknesses (curves with box or circle indicators), which may beobtained by multiplying the normalized scatter distribution shown inFIG. 8A by I_(bp). Also shown in FIG. 8B is the quantity I_(bp)·Sk₁(*)for the three slab thicknesses (solid-line curves). The fit is excellentover the thickness range of 0 to 10 cm. The Monte-Carlo simulations alsoprovide the values of I_(bp) for each of the thickness values, allowingan auxiliary thickness function T₁(I_(bp)) to be generated from thedata, such as by fitting a polynomial form to the data. As describedbelow in greater detail, auxiliary thickness function T₁(I_(bp)), andthe auxiliary thickness functions T₂(I_(bp)) and T₃(I_(bp)) describedbelow, can be useful in inventions of the present application thatallocate different instances of the scatter kernel to different portionsof the object based on thickness. Specifically, a kernel instance may beassigned to a thickness range, and the auxiliary thickness functions maybe used to determine if a pixel should be assigned to the kernelinstance based on its value of I_(bp).

FIG. 9A shows three Monte Carlo simulations of the normalized scatterdistributions from a perpendicular pencil beam (x_(m)=0, y_(n)=0)through four uniform rectangular slabs of water-based material (densityof 1 g/cc) at four different thicknesses: 10 cm, 15 cm, 20 cm, and 25cm. The distributions are normalized to I_(bp), are plotted as afunction of the radius r=[(x−x_(m))²+(y−y_(n))²]^(1/2), and are shown bydots in the figure. The value of I₀ for these simulations was set to atypical value found in cone-beam CT systems. FIG. 9A also shows a secondform Sk₂(*) of scatter kernel Sk(*) according to equation [8] fitted tothe scattered distributions (solid lines) by using a suitable set ofparameters A₂, B₂, α₂, β₂, σ_(1, 2), and σ_(2, 2). One exemplary set ofparameters for r, x_(m), and y_(n) in units of centimeters is:A₂=1.10×10⁻⁴, B₂=1.35, α₂=−0.173, β₂=0.978, σ_(1,2)=21.3, andσ_(2,2)=2.95. The fit is excellent over the thickness range of 10 to 25cm. FIG. 9B shows the actual scatter distribution for the four slabthicknesses (curves with circle indicators), which may be obtained bymultiplying the normalized scatter distribution shown in FIG. 9A byI_(bp). Also shown in FIG. 9B is the quantity I_(bp)·Sk₂(*) for the fourslab thicknesses (solid-line curves). The fit is excellent over thethickness range of 10 to 25 cm. The Monte-Carlo simulations also providethe values of I_(bp) for each of the thickness values, allowing athickness function T₂(I_(bp)) to be generated from the data, such as byfitting a polynomial form to the data.

FIG. 10A shows three Monte Carlo simulations of the normalized scatterdistributions from a perpendicular pencil beam (x_(m)=0, y_(n)=0)through four uniform rectangular slabs of water-based material (densityof 1 g/cc) at four different thicknesses: 30 cm, 35 cm, 40 cm, and 45cm. The distributions are normalized to I_(bp), are plotted as afunction of the radius r=[(x−x_(m))²+(y−y_(n))²]^(1/2), and are shown bydots in the figure. The value of I₀ for these simulations was set to atypical value found in cone-beam CT systems. FIG. 10A also shows asecond form Sk₃(*) of scatter kernel Sk(*) according to equation [8]fitted to the scattered distributions (solid lines) by using a suitableset of parameters A₃, B₃, α₃, β₃, σ_(1, 3), and σ_(2, 3). One exemplaryset of parameters for r, x_(m), and y_(n) in units of centimeters is:A₃=2.05×10⁻⁴, B₃=0.669, α₃=−0.270, β₃=0.421, σ_(1,3)=21.2, andσ_(2,2)=2.52. The fit is excellent over the thickness range of 30 to 50cm. FIG. 10B shows the actual scatter distribution for the four slabthicknesses (curves with circle indicators), which may be obtained bymultiplying the normalized scatter distribution shown in FIG. 10A byI_(bp). Also shown in FIG. 10B is the quantity I_(bp)·Sk₃(*) for thefour slab thicknesses (solid-line curves). The fit is excellent over thethickness range of 30 to 50 cm. The Monte-Carlo simulations also providethe values of I_(bp) for each of the thickness values, allowing athickness function T₃(I_(bp)) to be generated from the data, such as byfitting a polynomial form to the data.

As the parameters are different for each of the forms shown in FIGS.8-10, the forms can be denoted separately as Sk₁(*), Sk₂(*), and Sk₃(*),respectively. These forms will be used later to illustrate inventions ofthe present application.

For imaging devices 120 that have small dimensions relative to theirdistance to the object, the scatter profile I_(os) may be generated froma scatter kernel Sk(*) as follows:

$\begin{matrix}\begin{matrix}{{I_{os}\left( {x,y} \right)} = {\sum\limits_{m = {- M}}^{+ M}{\sum\limits_{n = {- N}}^{+ N}{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Sk}\left( {{x - x_{m}},{y - y_{n}},I_{bp}} \right)}}}}} \\{= {\sum\limits_{m = {- M}}^{+ M}{\sum\limits_{n = {- N}}^{+ N}{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)} \cdot}}}} \\{{S_{FF}\left( {{x - x_{m}},{y - y_{n}}} \right)}.}\end{matrix} & \lbrack 9\rbrack\end{matrix}$As equation [9] is based from equation [8A], both I_(bp) and S_(A)(I₀,I_(bp)) are functions of x_(m) and y_(n). The latter part of equation[9] may be recognized as the two-dimensional convolution of the product[I_(bp)S_(A)(*)] with S_(FF)(*), giving:I _(os)(x,y)=[I _(bp)(x,y)·S _(A)(I ₀ ,I _(bp))]

S _(FF)(x,y),  [10]where each of I_(bp) and S_(A)(I₀, I_(bp)) are now functions of x and yfor the convolution form. Since the Fourier transform of two convolvedfunctions is equal to the multiplication of their Fourier transforms,the profile I_(os) can be generated from the Fourier transforms asfollows:I _(os)(x,y)=ℑ⁻¹ {ℑ{I _(bp)(x,y)·S _(A)(I ₀ ,I _(bp))}·ℑ{S_(FF)(x,y)}}  [11]where ℑ{*} denotes a Fourier transform operation, and ℑ⁻¹{*} denotes aninverse Fourier transform. In equations [10] and [11], each of I₀,I_(bp), and S_(A)(I₀, I_(bp)) is a function of coordinates x and y(instead of the pencil beam coordinates x_(m) and y_(n)). In practice,the summation of equation [9] and the convolution of equation [10] aredone over the finite area of the pixel array, which means thatcontributions from pencil beams that terminate outside of the array arenot reflected in I_(os)(x,y). This can affect the scatter values at theedges of the array, and ways for compensating for finite-area summationare described below. It should be said that this is not a problem if theradiation shadow of the object falls within the pixel array. In thiscase, there is no scattering contribution from pencil beams thatterminate outside of the pixel array.

The Fourier transform of form factor S_(FF)(x,y) has primarilylow-frequency components because of its Gaussian form. As is known inthe mathematics art, the Fourier transform of a Gaussian is anotherGaussian, and Gaussian forms have primarily low-frequency components.Because of the summation in equation [9] and convolution in equation[10], the scattered radiation profile I_(os)(x,y) is relatively smooth(i.e., slowly varying) over the x,y domain, and its Fourier transformhas primarily low-frequency components. Accordingly, the Fouriertransforms in equation [11] can be generated from a decimated set ofdata (e.g., [I_(bp)(x,y)·S_(A)(I₀, I_(bp))]) on a coarse grid, whichconsiderably speeds the generation of the Fourier transform. As anexample, the values of [I_(bp)(x,y)·S_(A)(I₀, I_(bp))] associated with a1024 by 768 pixel array for a human torso may be decimated (reduced downto) a 104 by 30 array of “super pixels”, where the value of each superpixel represents the values of [I_(bp)(x,y)·S_(A)(I₀, I_(bp))] over a9-by-25 block or 10-by-26 block of real pixels. Each super pixel mayhave its location at the center of its respective block, and may begenerated from pixel values of [I_(bp)(x,y)·S_(A)(I₀,I_(bp))] within theblock, such as by generating an average of all or a portion of thevalues. Here, the term “average” has broad meaning where the “average”can be computed as a mean, a median, a strict average, or as any otherconventional central tendency value, and may include truncating thepixel values (i.e., removing pixel values at the distal ends of the datadistribution). The Fourier transform of the form factor, ℑ{S_(FF)(x,y)}, may be generated by using the values of the S_(FF)(x−x_(m),y−y_(n)) at the grid locations of the super pixels. The results of theinverse Fourier transform from equation [11] may be computed for thegrid locations of the super pixels, and the results of the inverseFourier transform may be interpolated/extrapolated onto the real pixelarray. The Fourier transforms and inverse Fourier transform used hereinmay be implemented by any of the known discrete Fourier transformprocesses. In addition, the Fourier transform of the form factorℑ{S_(FF)(x, y)} need only be generated once, and may be used for all ofprojections of the scan.

The array of super pixel blocks may be centered within the pixel array,leaving a margin of unused pixels around the array. This margin, incombination with the extrapolation of the inverse Fourier transform dataat the edges of the full array, can be used to overcome loss ofinformation caused by the fact that scatter contributions from thepencil beams that terminate outside the area of the pixel array are notreflected in the values of I_(os)(x,y) near the edges of the pixelarray. As another approach of overcoming the loss of information, theFourier transforms may be generated on an expanded array of super pixelsthat comprises the above-described array of super pixels for theinterior of the pixel array plus a band of “dummy” super pixels for themargin of unused pixels around the pixel array, and also optionally forareas outside of the pixel array surrounding the array. Values of[I_(bp)(x,y)·S_(A)(I₀,I_(bp))] at the super pixels may be extrapolatedonto the dummy super pixels, the Fourier transforms and inverse Fouriertransform may be generated and processed using the expanded array, andthe results of the inverse Fourier transform may be interpolated to thefull pixel array.

Referring briefly back to the imaging chain of FIG. 4B, one use of thesymmetric kernel is to estimate the individual profiles I_(o0) andI_(os) from their sum (I_(o0)+I_(os)). As described above, it waspossible to generate an estimate of the radiation profile I_(I) at thefront of the detector housing from the measured data from imaging device120, where I_(I)=(I_(o0)+I_(os)). Equation [11] can be used to estimatethe profile I_(os), and we can write the following relationship:I _(I)(x,y)=I _(o0)(x,y)+[I _(bp)(x,y)·S _(A)(I ₀ ,I _(bp))]

S _(FF)(x,y).  [12]In this use of the symmetric kernel, the coordinates x and y are known,and I₀(x,y) can be measured by a calibration step. However, I_(bp)(x,y)is not known; in fact, I_(bp)(x,y) is the profile I_(o0) that is to beestimated from the I_(I)(x,y). An iterative approach may be used todetermine I_(o0)(x,y) (and I_(bp)(x,y)) from equation [12].Specifically, an estimated value I_(EST) for I_(o0)(x,y) (andI_(bp)(x,y)) may be used in the convolution term of equation [12], and amore refined estimate of I_(o0)(x,y) may be generated as the differencebetween I_(I)(x,y) and the convolution term as follows:I _(o0)(x,y)=I _(I)(x,y)−[I _(EST)(x,y)·S _(A)(I ₀ ,I _(EST))]

S _(FF)(x,y).  [12A]The iterative approach is illustrated using one instance of the scatterkernel (an iterative approach using multiple instances of the scatterkernel will be illustrated later). An initial estimate I⁰ _(o0)(x,y) maybe generated for I_(o0)(x,y) and used to generate an initial value I¹_(EST)(x,y)=I⁰ _(o0)(x,y) and an initial value S_(A) ¹(I₀,I¹ _(EST)) forthe amplitude function S_(A)(I₀,I¹ _(EST)), where the superscriptsdenote the iteration number k=0, 1, 2, . . . K. The initial estimate I⁰_(o0)(x,y) may be a fraction of I_(I)(x,y), such as I_(I)(x,y)/2, or theestimate for I_(o0)(x,y) generated for a prior projection in the objectscan. An initial estimate of the product[I_(EST)(x,y)·S_(A)(I₀,I_(EST))] may be generated as [I¹_(EST)(x,y)·S_(A) ¹(I₀,I_(EST))]. Next, a first iteration estimate I¹_(o0)(x,y) may be generated for I_(o0)(x,y) by applying equation [12A]as follows:I ¹ _(o0)(x,y)=I _(I)(x,y)−[I ¹ _(EST)(x,y)·S _(A) ¹(I ₀ ,I ¹ _(EST))]

S _(FF)(x,y),  [13]where the convolution may be performed using Fourier transforms asdescribed above. More refined estimates of I_(EST)(x,y) and S_(A)(I₀,I_(EST)) for the convolution term may then be generated from I¹_(o0)(x,y), and equation [13] may be reiterated to further refine theestimate for I_(o0)(x,y). The following iteration approach may be usedfor subsequent iterations k=2 to K for generating refined estimatesI^(k) _(o0)(x,y):I ^(k) _(EST)(x,y)=0.5*I ^(k-1) _(o0)(x,y)+0.5*I ^(k-2)_(o0)(x,y)  [14A]I ^(k) _(o0)(x,y)=I _(I)(x,y)−[I ^(k) _(EST)(x,y)·S _(A) ^(k)(I ₀ ,I^(k) _(EST))]

S _(FF)(x,y)  [14B]Equation [14A] provides some damping on the rate of change of I^(k)_(EST)(x,y) and S_(A) ^(k)(I₀,I^(k) _(EST)) between the iterationscompared to the selection of I^(k) _(EST)(x,y)=I^(k-1) _(o0)(x,y) andS_(A) ^(k)(I₀,I^(k) _(EST))=S_(A)(I₀, I^(k-1) _(o0)), and minimizes thechances of the values of I^(k) _(EST)(x,y) and S_(A) ^(k)(I₀, I^(k)_(EST)) oscillating during the iterations. An equal weighting factor of0.5 has been used in equation [14A], but different weighting factors maybe used (at least the first weighting factor should be non-zero and bothweighting factors should sum to 1). Equations [13] and [14] may beapplied to the real pixel array, or to the super pixel array. In thelatter case, the super pixel values for I^(k) _(o0)(x,y) and I_(I)(x,y)may be generated as averages in the manner described above for thequantity [I_(bp)(x,y)·S_(A)(I₀, I_(bp))], and, if the Fourier transformprocess has been used to generate [I^(k) _(EST)(x,y) S_(A) ^(k)(I₀,I^(k) _(EST))]

S_(FF)(x,y) with decimated data, the results of the inverse Fouriertransform do not need to be interpolated/extrapolated. The iterationsmay be continued until the change between iteration values of [I^(k)_(EST)(x,y)·S_(A) ^(k)(I₀,I^(k) _(EST))] and [I^(k-1) _(EST)(x,y)·S_(A)^(k-1)(I₀, I^(k-1) _(EST))] is below a desired (e.g., set) amount for adesired (e.g., set) number of pixels or super pixels, or until thechange between iteration values of I^(k) _(o0)(x,y) and I^(k-1)_(o0)(x,y) is below a desired amount for a desired number of pixels orsuper pixels. Additional criteria or other criteria may be used todetermine when to stop the iterations, such as monitoring an averagemeasure of the changes over the pixels or super pixels, and setting anupper limit to the number of iterations. If the iterations have beencarried out on the super pixels, then one application of equation [14B]using the real pixel array may be used to generate I^(k) _(o0)(x,y)values over the real pixel array after the last iteration (k=K). Forthis, the convolution term may be generated on the super pixel array andthen interpolated and extrapolated onto the real pixel array. Thescattered radiation at any iteration, including the last iteration k=K,may be generated as:I ^(k) _(os)(x,y)=[I ^(k) _(EST)(x,y)·S _(A) ^(k)(I ₀ ,I ^(k) _(EST))]

S _(FF)(x,y)  [14C-1]or as:I ^(k) _(os)(x,y)=[I ^(k) _(o0)(x,y)·S _(A) ^(k)(I ₀ ,I ^(k) _(o0))]

S _(FF)(x,y).  [14C-2]

Another exemplary iteration approach for subsequent iterations k=2 to Kis as follows:

$\begin{matrix}{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {15A} \right\rbrack \\{\mspace{79mu}{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\lbrack {{{I_{EST}^{k}\left( {x,y} \right)} \cdot {{S_{A}^{k}\left( {I_{0},I_{EST}^{k}} \right\rbrack} \otimes {S_{FF}\left( {x,y} \right)}}},} \right.}}} & \left\lbrack {15B} \right\rbrack\end{matrix}$with [I¹ _(EST)(x,y)·S_(A) ¹(I₀I¹ _(EST))]=[I⁰ _(o0)(x,y)·S_(A)(I₀, I⁰_(o0))]. Equation [15A] generates a scatter estimate for the currentiteration k as a weighted average of the scatter as generated from themost recent estimate of I_(o0) (which is I^(k-1) _(o0)(x,y)) and thescatter used in the last iteration. Like Equation [14A], equation [15A]provides some damping on the rate of change of [I^(k) _(EST)(x,y)·S_(A)^(k)(I₀,I^(k) _(EST))] between the iterations, and minimizes the chancesof the values [I^(k) _(EST)(x,y)·S_(A) ^(k)(I₀,I^(k) _(EST))]oscillating during the iterations. An equal weighting factor of 0.5 hasbeen used in equation [15], but different weighting factors may be used(at least the first weighting factor should be non-zero and bothweighting factors should sum to 1). Equations [13] and [15] may beapplied to the real pixel array, or to the super pixel array. In thelatter case, the super pixel values for I^(k) _(o0)(x,y) and I_(I)(x,y)may be generated as averages in the manner described above for thequantity [I_(bp)(x,y)·S_(A)(I₀, I_(bp))], and, if the Fourier transformprocess has been used to generate [I^(k) _(EST)(x,y)·S_(A) ^(k)(I₀,I^(k) _(EST))]

S_(FF)(x, y) with decimated data, the results of the inverse Fouriertransform do not need to be interpolated/extrapolated. The abovepossible criteria for determining when to end the iterations of equation[14] may be used to determine when to end the iterations of equation[15]. If the iterations have been carried out on the super pixels, thenone application of equation [15B] using the real pixel array may be usedto generate I^(k) _(o0)(x,y) values over the real pixel array. For this,the convolution term may be generated on the super pixel array and theninterpolated and extrapolated onto the real pixel array. The scatteredradiation at any iteration, including the last iteration k=K, may begenerated by the above equations [14C].

In equations [14B] and [15B], the term [I^(k) _(EST)(x,y)·S_(A) ^(k)(I₀,I^(k) _(EST))]

S_(FF)(x, y) is preferably generated by the Fourier transform method,but can be generated by a full-space convolution using equation [10] orby a full-space summation using equation [9].

Skew Correction.

The accuracy of equation [9] degrades somewhat at the edges of theimaging area in the case where the dimensions of an imaging device 120are not small relative to distance between the imaging device and theobject. This is because the radiation pencil beams at the edges makeoblique angles to the surface of the imaging device 120, up to 9° intypical systems with imaging device 120 centered about the projectionaxis, and up to 14.5° in typical systems with device 120 offset from theprojection axis for half-fan scanning. This causes the scatteredradiation to strike the imaging device 120 at skewed angles, asillustrated in FIG. 11. In the figure, two pencil beams are shown. Beam1 is perpendicular to the detector plane of imaging device 120, and isdirected to pixel location (x,y)=(0,0). Beam 2 is oblique, is directedto pixel location (x,y)=(x_(n),y_(m)), and makes an angle θ_(p) withrespect to the projection axis, an angle θ_(xm) as projected onto thex-z plane, and an angle θ_(yn) as projected onto the y-z plane. There isno skewing of the scattered radiation from Beam 1, but there is someskewing from the scattered radiation from Beam 2. The peak of thescatter distribution for Beam 2 is less than that for Beam 1 by a factorof cos³(θ_(p)) because of two factors. As the first factor, the distanced′ between the detector and the object along Beam 2 is longer than thedistance d between the detector and the object along Beam 1 by an amountcos(θ_(p)), which means that the radiation intensity decreases by afactor of cos²(θ_(p)) because the inverse square law of sphericalradiation propagation. As the second factor, the scatter radiationstrikes the pixel at an angle of θ_(p) rather than perpendicular angle,which reduces the intensity by another factor of cos(θ_(p)). As afurther skewing effect, the right side of the scatter distribution iscompressed toward the distribution's peak and increased in relativevalue since it strikes the detector plane before the peak does, and theleft side of the scatter distribution is expanded away from the peak anddecreased in relative value since it strikes the detector plane afterthe peak does. Mathematically modeling of the compression and expansionof the sides is difficult and only yields small corrections. However,the relative increase and decrease in the values of the sides can bereadily modeled by the term:d ² /[d+tan(θ_(xm))(x−x _(m))+tan(θ_(yn))·(y−y _(n))]².With the above observations, the following correction factor Z(x−x_(m),y−y_(n), θ_(p), θ_(xm), θ_(yn)) may be used to remove a substantialportion of the skewing effects:

$\begin{matrix}{{{Z\left( {{x - x_{m}},{y - y_{n}},\theta_{p},\theta_{xm},\theta_{yn}} \right)} = {{\cos^{3}\left( \theta_{P} \right)} \cdot \left( \frac{d}{d + {{\tan\left( \theta_{xm} \right)} \cdot \left( {x - x_{m}} \right)} + {{\tan\left( \theta_{yn} \right)} \cdot \left( {y - y_{n}} \right)}} \right)^{2}}},} & \lbrack 16\rbrack\end{matrix}$where cos(θ_(p))=D/[D²+x_(m) ²+y_(n) ²]^(1/2), tan(θ_(xm))=x_(m)/D,tan(θ_(yn))=y_(n)/D.

Correction factor Z(x−x_(m), y−y_(n), θ_(p), θ_(xm), θ_(yn)) is appliedby multiplying it with Sk(x−x_(m), y−y_(n), I_(bp)) in equation [9].Unfortunately, the form of Z(*) in equation [16] does not lead to aready convolution form like equation [10]. This is because tan(θ_(xm))and tan(θ_(yn)), which multiply the convolution differences (x−x_(m))and (y−y_(n)) and change in value with x_(m) and y_(n), cannot beregrouped with [I_(bp)(x,y)·S_(A)(I₀, I_(bp))]. One approach is to usean approximate correction factor Z′(θ_(p))=cos³(θ_(p)), which removesthe mixed products and the dependence on x−x_(m), y−y_(n), θ_(xm), andθ_(yn). The approximate correction factor Z′(θ_(p)) can be grouped with[I_(bp)(x,y)·S_(A)(I₀, I_(bp))] in equation [9], and leads to thefollowing modified form of the convolution equation:

$\begin{matrix}\begin{matrix}{{I_{os}\left( {x,y} \right)} = {\left\lbrack {{Z^{\prime}\left( \theta_{xy} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}}} \\{{= {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A}^{\prime}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}}},}\end{matrix} & \left\lbrack {10A} \right\rbrack\end{matrix}$where S_(A)′(I₀, I_(bp))=Z′(θ_(xy))·S_(A)(I₀, I_(bp)), and where theargument to Z′(*) is now θ_(xy) instead of θ_(p) because of theconvolution form (cos(θ_(xy))=D/[D²+x²+y²]^(1/2)). The generation ofI_(os)(x,y) using the Fourier transforms of equation [11] then directlyfollows using S_(A)′(I₀,I_(bp)) in place of S_(A)(I₀,I_(bp)), asfollows:I _(os)(x,y)=ℑ⁻¹ {ℑ{I _(bp)(x,y)·S _(A)′(I ₀ ,I _(bp))}·ℑ{S_(FF)(x,y)}}.  [11A]The iteration approaches illustrated by equations [13]-[15] alsodirectly follow using I_(EST) in place of I_(bp) and S_(A)′(I₀, I_(EST))in place of S_(A)(I₀, I_(EST)).

To provide another approximate correction factor that can lead to aconvolution form, Z(x−x_(m), y−y_(n), θ_(p), θ_(xm), θ_(yn)) may belinearized to provide:

${Z^{''}\left( {{x - x_{m}},{y - y_{n}},\theta_{p},\theta_{xm},\theta_{yn}} \right)} = {{\cos^{3}\left( \theta_{P} \right)} \cdot {\left( {1 - {2 \cdot \frac{\tan\left( \theta_{xm} \right)}{d} \cdot \left( {x_{m} - x} \right)} - {2 \cdot \frac{\tan\left( \theta_{yn} \right)}{d} \cdot \left( {y_{n} - y} \right)}} \right).}}$This enables I_(os)(x,y) to be generated from the following sum of threeconvolutions:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\left\lbrack {{\cos^{3}\left( \theta_{xy} \right)} \cdot I_{bp} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes \left\lbrack {S_{FF}\left( {x,y} \right)} \right\rbrack} - {\quad{{\left\lbrack {2 \cdot {\cos^{3}\left( \theta_{xy} \right)} \cdot I_{bp} \cdot {\tan\left( \theta_{x} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes \left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {x/d} \right)} \right\rbrack} - {\quad{{\left\lbrack {2 \cdot {\cos^{3}\left( \theta_{xy} \right)} \cdot I_{bp} \cdot {\tan\left( \theta_{y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes \left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {y/d} \right)} \right\rbrack},}}}}}} & \left\lbrack {10B} \right\rbrack\end{matrix}$where θ_(p), θ_(xm), θ_(yn) have been replaced by θ_(xy), θ_(x), θ_(y),respectively, because of the convolution form (tan(θ_(x))=x/D,tan(θ_(y))=y/D). I_(os)(x,y) may be generated by Fourier transforms bytaking the Fourier transforms of each of the bracketed terms (6 intotal), multiplying the transforms corresponding to the convolutionpairs to generate three multiplication products, summing the threemultiplication products together, and taking the inverse Fouriertransform of the summation. The iteration approaches illustrated byequations [13]-[15] also directly follow using I_(EST) in place ofI_(bp), and using S_(A)′(I₀, I_(EST)) in place of S_(A)(I₀, I_(EST))

Multiple Symmetric Kernel Forms for Different Thickness Ranges.

As indicated above, another invention set of the present applicationrelates to constructing two or more instances of the new symmetric formfor two or more respective ranges of object thickness, and applying theinstances of the new symmetric form to regions of a radiographicprojection according to a measure of the object's thickness in theregions. An exemplary embodiment of the invention is illustrated withthe kernel form instances Sk₁(*), Sk₂(*), and Sk₃(*) and theircorresponding thickness functions T₁(*), T₂(*), and T₃(*) previouslydescribed with respect to FIGS. 8-10. The thickness functions can beused to determine the values of radiation intensity (I_(bp)) thatseparate the thickness regions from one another. For example, it can befound from the thickness functions that an intensity value I_(A)separates the first thickness region (0-10 cm, modeled by kernel Sk₁(*))from the second thickness region (10-25 cm, modeled by kernel Sk₂(*)),and that an intensity value I_(B) separates the second thickness regionfrom the third thickness region (25 cm and above, modeled by Sk₃(*)),with I_(A) being greater than I_(B). During each iteration of either ofequations [14] or [15], estimated primary beam profile I^(k) _(EST)(x,y)can be determined from either the initial value or the results of theprevious iteration. Each pixel value of I^(k) _(EST)(x,y) (or I_(bp))may be compared with I_(A) and I_(B) to determine which kernel should beused to generate the scattered radiation associated with the pencil beamterminating at the pixel. FIG. 12 shows a top plan representation of thepixel array during an iteration k where the pixels are divided into fourgroups according to object thickness. Two groups are labeled with thenotation “1” and cover pixels with primary beam intensity values I_(bp)or I^(k) _(EST)(x,y) greater than I_(A), corresponding to an objectthickness range from 0 cm up to 10 cm. These pixels are modeled with thefirst kernel instance Sk₁(*). One group is labeled with the notation “2”and covers pixels with intensity values I_(bp) or I^(k) _(EST)(x,y)equal to or less than I_(A) and greater than I_(B), corresponding to anobject thickness range from 10 cm up to 25 cm. These pixels are modeledwith the second kernel instance Sk₂(*). The last group is labeled withthe notation “3” and covers pixels with intensity values I_(bp) or I^(k)_(EST)(x,y) equal to or less than I_(B), corresponding to an objectthickness range of 25 cm or more. These pixels are modeled with thethird kernel instance Sk₃(*).

The summation-based equation [9] for I_(os)(x,y) may be expanded toinclude three separate double summations, one for each of the kernelinstances: Sk₁(*), Sk₂(*), and Sk₃(*). Each double summation may be onlyover the group of pixels covered by the kernel of the double summation.The expanded summation-based equation is as follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\sum\limits_{{Group}\mspace{14mu} 1}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Sk}_{1}\left( {{x - x_{m}},{y - y_{n}},I_{bp}} \right)}}}} + {\sum\limits_{{Group}\mspace{14mu} 2}{\sum{{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {Sk}_{2}}\left( {{x - x_{m}},{y - y_{n}},I_{bp}} \right)}}} + {\sum\limits_{{Group}\mspace{14mu} 3}{\sum{{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {Sk}_{3}}{\left( {{x - x_{m}},{y - y_{n}},I_{bp}} \right).}}}}}} & \lbrack 17\rbrack\end{matrix}$As equation [17] is based from equation [8A], both I_(bp) and S_(A)(I₀,I_(bp)) are functions of x_(m) and y_(n). Any of the skew correctionfactors Z(*), Z′(*), or Z″(*) may be applied to equation [17] bymultiplying it with each of the kernel instances. To obtain thecorresponding convolution form for I_(os)(x,y), each kernel instance maybe written as the product of its amplitude function and its form factor,e.g., Sk₁(x−x_(m), y−y_(n), I_(bp))═S_(A,1)(I₀, I_(bp))·S_(FF,1)(x, y),and each amplitude function may be set to a value of zero for pixelsthat are not covered by its kernel instance. For example, amplitudefunction S_(A,1)(I₀, I_(bp)) for Sk₁(*) has zero values for the pixellocations in Groups 2 and 3, and non-zero values for the pixels in Group1; amplitude function S_(A,2)(I₀,I_(bp)) for Sk₂(*) has zero values forthe pixel locations in Groups 1 and 3, and non-zero values for thepixels in Group 2, etc. With that format of the amplitude functions, theconvolution form is readily identified as:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}}} & \lbrack 18\rbrack\end{matrix}$where each of I_(bp), S_(A,1)(I₀, I_(bp)), S_(A,2)(I₀, I_(bp)), andS_(A,3)(I₀, I_(bp)) are now functions of x and y for the convolutionform. If there are no pixels or super pixels in the third group, thenthe convolution [I_(bp)(x,y)·S_(A,3)(I₀,I_(bp))]

S_(FF,3)(x, y) may be omitted from equation [18] and other similarequations provided below. Skew correction factors Z′(θ_(p)) may beapplied to equation [18] by multiplying it with each of the amplitudefunctions, as was done with equation [10A]. The application of skewcorrection factors Z″(*) to equation [18] follows a similar form to thatof equation [10B], but leads to the summation of nine convolution terms.

Since the Fourier transform of two convolved functions is equal to themultiplication of their Fourier transforms, the profile I_(os) fromequation [18] can be generated from the Fourier transforms as follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{??}^{- 1}{\left\{ {{{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},1}\left( {x,y} \right)} \right)}} + {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},2}\left( {x,y} \right)} \right)}} + {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},3}\left( {x,y} \right)} \right)}}} \right\}.}}} & \lbrack 19\rbrack\end{matrix}$Equation [19] is typically applied on the array of super pixels ratherthan the array of real pixels, using the same super pixel constructionprocesses as described above with regard to equation [11]. In thisregard, an average value of I_(bp) over the pixels covered by a superpixel may be used to determine which kernel instance to use for thesuper pixel. The Fourier transform of the form factors ℑ{S_(FF,1)(x,y)}, ℑ{S_(FF,2)(x, y)}, and ℑ{S_(FF,3)(x, y)} may be generated by usingthe values of the form factors at the grid locations of the superpixels; they may be generated once, and may be used for all of theprojections of the scan. The results of the inverse Fourier transformfrom equation [19] may be computed for the grid locations of the superpixels, and the results of the inverse Fourier transform may beinterpolated/extrapolated onto the real pixel array in any of theabove-described manners. The Fourier transforms and inverse Fouriertransform used herein may be implemented by any of the known discreteFourier transform processes. If there are no pixels or super pixels inthe third group, then the Fourier productℑ(I_(mn)·S_(A,3)(I₀,I_(mn)))·ℑ(S_(FF,3)(x, y)) may be omitted fromequation [19], and other similar equations provided below.

Using equation [18], the corresponding form of equation [12A] forgenerating an estimate of I_(o0)(x,y) from I_(I)(x,y) is as follows:

$\begin{matrix}{{I_{o\; 0}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\{ {{\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left. \quad{\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}.}} \right.}} & \lbrack 20\rbrack\end{matrix}$An iteration approach similar to equations [13]-[15] may used togenerate an estimate of I_(o0)(x,y). An initial estimate for I⁰_(o0)(x,y) may be generated for I_(o0)(x,y), such as using a fraction ofI_(I)(x,y), and used for I_(EST)(x,y) in equation [20]. Each pixel valueof the initial estimate I⁰ _(o0)(x,y) (I_(EST)(x,y)) may be comparedwith the intensity values I_(A) and I_(B) to determine which kernelinstance the pixel should be assigned to, as described above. Then, eachkernel instance's amplitude function S_(A,1)(I₀,I_(EST)),S_(A,2)(I₀,I_(EST)), and S_(A,3)(I₀,I_(EST)) is generated using thepixel values of I⁰ _(o0)(x,y) (I_(EST)(x,y)) of the pixels that havebeen assigned to the kernel instance. Next, a first iteration estimateI¹ _(o0)(x,y) may be generated for I_(o0)(x,y) by applying equation [20]as follows, using our previous notation I¹ _(EST) for the firstiteration:

$\begin{matrix}{{I_{o\; 0}^{1}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\{ {{\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,1}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,2}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,3}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\}}} & \lbrack 21\rbrack\end{matrix}$where the convolution may be performed using Fourier transforms asdescribed above with reference to equation [19]. More refined estimatesof I_(EST)(x,y), S_(A,1)(I₀, I_(EST)), S_(A,2)(I₀, I_(EST)), andS_(A,3)(I₀, I_(EST)) may then be generated from I¹ _(o0)(x,y), andequation [21] may be reiterated to further refine the estimate forI_(o0)(x,y). During this refinement, it is possible for there to bechanges in the kernel assignments for some of the pixels (or superpixels). The following iteration approach similar to equation [14] maybe used for subsequent iterations k=2 to K for generating refinedestimates I^(k) _(o0)(x,y):

$\begin{matrix}{\mspace{79mu}{{{I_{EST}^{k}\left( {x,y} \right)} = {{0.5*{I_{o\; 0}^{k - 1}\left( {x,y} \right)}} + {0.5*{I_{o\; 0}^{k - 2}\left( {x,y} \right)}}}},}} & \left\lbrack {22A} \right\rbrack \\{\mspace{79mu}{{{Generate}\mspace{14mu}{S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}},{S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)},{{and}\mspace{14mu}{S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}},}} & \left\lbrack {22B} \right\rbrack \\{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\{ {{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left. \quad{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}.}} \right.}} & \left\lbrack {22C} \right\rbrack\end{matrix}$Equation [22A] provides some damping on the rate of change of I^(k)_(EST)(x,y) and the amplitude functions S_(A,1) ^(k)(I₀, I^(k) _(EST)),S_(A,2) ^(k)(I₀,I^(k) _(EST)), and S_(A,3) ^(k)(I₀, I^(k) _(EST))between the iterations, and minimizes the chances of the function valuesoscillating during the iterations. An equal weighting factor of 0.5 hasbeen used in equation [22A], but different weighting factors may be used(at least the first weighting factor should be non-zero and bothweighting factors should sum to 1). Also, the kernel-instance assignmentof the pixels (or super pixels) may be frozen for the latter iterationsto prevent oscillations caused by switching assignments at groupboundaries.

Equations [21] and [22] may be applied to the real pixel array, or tothe super pixel array. In the latter case, the super pixel values forI^(k) _(o0)(x,y) and I_(I)(x,y) may be generated as averages in themanner described above for equations [13]-[15], and, if the Fouriertransform process (equation [19]) has been used to generate theconvolution terms from decimated data, the results of the inverseFourier transform do not need to be interpolated/extrapolated during theiterations. The iterations may be continued until the change between thecurrent and previous values of each amplitude function, as multiplied byI^(k) _(EST)(x,y), is below a desired (e.g., set) amount for a desired(e.g., set) number of pixels or super pixels, or until the changebetween iteration values of I^(k) _(o0)(x,y) and I^(k-1) _(o0)(x,y) isbelow a desired amount for a desired number of pixels or super pixels.Additional criteria or other criteria may be used to determine when tostop the iterations, as described above for equations [13]-[15]. If theiterations have been carried out on the super pixels, then oneapplication of equation [22C] using the real pixel array may be used togenerate I^(k) _(o0)(x,y) values over the real pixel array after thelast iteration (k=K). For this, the convolution term may be generated onthe super pixel array and then interpolated and extrapolated onto thereal pixel array. The scattered radiation at any iteration, includingthe last iteration k=K, may be generated as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = {{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}}} & \left\lbrack {22D\text{-}1} \right\rbrack\end{matrix}$or as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = {{\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {{S_{{FF},3}\left( {x,y} \right)}.}}}} & \left\lbrack {22D\text{-}2} \right\rbrack\end{matrix}$

Another iteration approach similar to that of equation [15] forsubsequent iterations k=2 to K is as follows:

$\begin{matrix}{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,1}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {23A\text{-}1} \right\rbrack \\{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,2}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {23A\text{-}2} \right\rbrack \\{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,3}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {23A\text{-}3} \right\rbrack \\{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\{ {{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\}}} & \left\lbrack {23B} \right\rbrack\end{matrix}$with [I¹ _(EST)(x,y)·S_(A,1) ¹(I₀,I¹ _(EST))]=[I⁰ _(o0)(x,y)·S_(A,1)(I₀,I⁰ _(o0))], [I¹ _(EST)(x,y)·S_(A,2) ¹(I₀,I¹ _(EST))]=[I⁰_(o0)(x,y)*S_(A,2)(I₀, I⁰ _(o0))], and [I¹ _(EST)(x,y)·S_(A,3) ¹(I₀,I¹_(EST))]=[I⁰ _(o0)(x,y)·S_(A,3)(I₀, I⁰ _(o0))]. Each of equation [23A]generates a scatter estimate for the current iteration k as a weightedaverage of the scatter as generated from the most recent estimate ofI_(o0) (which is I^(k-1) _(o0)(x,y)) and the scatter used in the lastiteration. Like Equation [22A], equations [23A] provide some damping onthe rate of change of the amplitude functions between the iterations,and minimize the chances of the function values oscillating during theiterations. An equal weighting factor of 0.5 has been used in equation[23A], but different weighting factors may be used (at least the firstweighting factor should be non-zero and both weighting factors shouldsum to 1). If the kernel assignment of a pixel or super pixel changesduring an iteration, its I_(EST)(*)·S_(A,X)(*) value will be averagedwith zero values for subsequent iterations if S_(A,X)(*) changes from anon-zero value to a zero value, and its I_(EST)(*)·S_(A,X)(*) value willstart from zero and be averaged with non-zero value for subsequentiterations if S_(A,X)(*) changes from a zero value to a non-zero value,thereby providing a smooth transition. Equations [21] and [23] may beapplied to the real pixel array, or to the super pixel array. In thelatter case, the super pixel values for I^(k) _(o0)(x,y) and I_(I)(x,y)may be generated as averages in the manner described above for equations[13]-[15], and, if the Fourier transform process (equation [19]) hasbeen used to generate the convolution terms from decimated data, theresults of the inverse Fourier transform do not need to beinterpolated/extrapolated during the iterations. The above possiblecriteria for determining when to end the iterations of equation [22] maybe used to determine when to end the iterations of equation [23]. If theiterations have been carried out on the super pixels, then oneapplication of equation [23B] using the real pixel array may be used togenerate I^(k) _(o0)(x,y) values over the real pixel array. For this,the convolution term may be generated on the super pixel array and theninterpolated and extrapolated onto the real pixel array. The scatteredradiation at any iteration, including the last iteration k=K, may begenerated according to equations [22D].

In equations [22C] and [23B], the convolution terms are preferablygenerated by the Fourier transform method, but they can be generated bya full-space convolution using equation [18] or by a full-spacesummation using equation [17]. These implementations, though slow, wouldallow use of the full skew correction factor Z(θ_(p)). Skew correctionfactors Z′(θ_(p)) may be applied to each of equations [21], [22C] and[23B] by multiplying it with each of the amplitude functions, as wasdone with equation [10A]. The application of skew correction factorsZ″(*) to each of equations [21], [22C] and [23B] follows a similar formto that of equation [10B], but leads to the summation of nineconvolution terms, but which can be readily handled by the Fouriertransform form of equation [19].

Asymmetric Point-Scatter Functions.

As indicated above, yet another set of inventions of the presentapplication relates to a new class of asymmetric point-scatter functionsthat provide improved projection-correction results relative tosymmetric point-scatter functions. The new class of asymmetricpoint-scatter functions, also called asymmetric kernels, was discoveredby the inventors through experiments on slabs of material withnon-uniform thicknesses. FIG. 13 shows a conventional Monte-Carlo-basedexperiment of directing a pencil beam of radiation at a perpendicularangle through a material slab (e.g., water, 1 g/cc density) having auniform thickness of 300 mm (30 cm). The resulting profile of scatteredradiation is shown in FIG. 15 by the curve with square indicators; ithas radial symmetry about the axis of the pencil beam. In reality,objects are not rectangular, particularly human objects. FIG. 14 showsan innovative Monte-Carlo-based experiment by the inventors of directinga radiation pencil beam through a material slab (e.g., water, 1 g/ccdensity) having a wedge shape. The wedge has an angle of 45 degrees atits left side, and two angles of 67.5 degrees at its right side. Thewedge has a symmetry axis passing through the 45-degree wedge angle, asshown in the figure. The pencil beam axis is perpendicular to thewedge's symmetry axis, is directed at point EP of the wedge, and passesthrough a portion of the wedge having a thickness of 300 mm (30 cm). Theresulting profile of scattered radiation is shown in FIG. 15 by thecurve with triangle indicators; it is asymmetric about the axis of thepencil beam. Relative to the symmetric curve, the asymmetric curve iselevated through the thinner region of the object and depressed throughthe thicker region of the object. This is because the scatteredradiation sees less attenuation when it passes through the thinnerregion of the wedge, and is therefore elevated in magnitude relative tothe uniform-slab case, and sees more attenuation when it passes throughthe thicker region of the wedge, and is therefore depressed in magnituderelative to the uniform-slab case. Thus, the scatter intensity changeswith the thickness variations in the neighborhood of the pencil beam,even if the path length seen by the pencil beam itself is the same inboth the wedge and uniform slab cases.

Over a range of object thicknesses, this asymmetric effect for aradiation pencil beam can be modeled by what we call an asymmetrickernel Ak(*). Ideally, the asymmetric kernel is a function of the point(x,y) where the scattered radiation terminates on device 120 and thepoint (x_(m),y_(n)) where the pencil beam terminates on device 120, justas in the case of the symmetric kernels, and is also a function of theobject thickness that the scattered radiation sees in traveling frompoint EP to point (x,y). In the context of the tomographicreconstruction, an object thickness τ(x,y) can be associated with eachpoint (x,y) of imaging device 120, and can be estimated from themeasured projection data according to the relation τ(x,y)=ln[I₀/I_(o0)(x,y)]/μ, where μ is around 0.19 to 0.21 cm⁻¹ for water- andsoft-tissue like materials (density of ˜1 g/cc) irradiated by radiationwith typical beam energies. However, τ(x,y) is the object thickness seenalong the direct path from source 110 to the point (x,y), not the objectthickness seen by the scattered radiation traveling from point EP topoint (x,y). The scattered radiation to point (x,y) sees an objectthickness that is between τ(x,y) and the object thickness seen by thepencil beam, the latter of which can be denoted at τ(x_(m),y_(n)). Forthe wedge shape, the inventors have found that the object thickness seenby the scattered radiation can be estimated as follows:

$\begin{matrix}{{\tau\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)},} & \lbrack 24\rbrack\end{matrix}$where M is a factor of about 1.6 that is due to the difference in thelengths between the path from source 110 to point (x,y) and the pathfrom point EP to point (x,y). Factor M is dependent upon the systemconfiguration.

Referring to FIG. 15, we propose to multiply the scatter distributionfound in the uniform slab case (the curve with square indicators) by thefollowing asymmetric form factor that uses the form of equation [24]:

$\begin{matrix}{{{Aff}\left( {x_{m},y_{n},{x_{,}y}} \right)} = \left\{ {1 + {\gamma \cdot \left\lbrack {{\tau\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)} - {\tau\left( {x_{m},y_{n}} \right)}} \right\rbrack}} \right\}} & \lbrack 25\rbrack\end{matrix}$where γ is a constant having a value of approximately −0.256·μ for thewedge object example, where μ is around 0.19 to 0.21 cm⁻¹ for water- andsoft-tissue like materials (density of ˜1 g/cc) irradiated by radiationwith typical beam energies. The unity term (“1”) in equation [25] modelsthe uniform slab case, while the remaining term in equation [25]provides a variation based on object thickness that the scatteredradiation sees as it travels to point (x,y) relative to the objectthickness that it would see in the uniform slab case, the latter ofwhich can be modeled by τ(x_(m),y_(n)) since Aff(x_(m), y_(n), x, y) hasbeen constructed to be multiplied with the scatter distribution found inthe uniform slab case. The resulting curve is shown as a solid line(with no line indicators) in FIG. 15. The curve is very close to theprofile from the Monte Carlo simulation of the wedge object case (thecurve with triangle indicators), and is an excellent fit.

A general thickness function T_(I)(I₀(x,y), I_(bp)(x,y)) may beconstructed from measured empirical data; it may have a form as simpleas T_(I)(I₀, I_(bp))=ln {I₀(x,y)/I_(bp)(x,y)}/μ, where μ is around 0.19to 0.21 cm⁻¹ for water- and soft-tissue like materials (density of ˜1g/cc) irradiated by radiation with typical beam energies. T_(I)(*) maybe used in place of τ(x,y) and τ(x_(m),y_(n)) in Equation [25] with goodresults. If I₀(x,y) is uniform or slowly varying, we may use thedivision property of logarithms to derive the following additionalasymmetric form factor from equation [25]:

$\begin{matrix}{{{Aff}^{\;{Ibp}}\left( {x_{m},y_{n},x,y} \right)} = \left\{ {1 + {\left( {\gamma/\mu} \right) \cdot \left\lbrack {{\ln\left( {I_{bp}\left( {x_{m},y_{n}} \right)} \right)} - {\ln\left( {I_{bp}\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)} \right)}} \right\rbrack}} \right\}} & \left\lbrack {25A} \right\rbrack\end{matrix}$The use of equation [25] and the use of T_(I)(*) in place of τ(x,y) inEquation [25] captures not only thickness variations, but also densityvariations in the object, which also can have asymmetric effects on thescattering. Thus, these forms are believed to be more general.

As it turns out, the factor M in equation [25] poses a challenge tousing convolutions and Fourier transforms to generate the summation ofthe scatter kernels over the area of imaging device 120. The challengecan be readily overcome by using a value of M=1 instead of M=1.6, andadjusting the value of γ, which does not significantly degrade the fit.As another approach, it can be generally assumed that the objectthickness varies in a piecewise linear manner from point (x,y) to point(x_(m),y_(n)), and equation [24] can be approximated as follows:

$\begin{matrix}{{{\tau\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)} = {{\tau\left( {{\frac{x}{M} + {\frac{M - 1}{M} \cdot x_{m}}},{\frac{y}{M} + {\frac{M - 1}{M} \cdot y_{n}}}} \right)} \approx {{\frac{1}{M} \cdot {\tau\left( {x,y} \right)}} + {\frac{M - 1}{M} \cdot {\tau\left( {x_{m},y_{n}} \right)}}}}},} & \lbrack 26\rbrack\end{matrix}$which can be simply viewed as a ratio mixing of the thickness valuesbased on the ratio mixing of the coordinates in the arguments ofequation [24]. Equation [26] can be formally derived as follows. Forsimplicity, denote the point

$\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)$as point (x′,y′), and denote its thickness as τx′,y′). Points (x,y) and(x′,y′) and their thickness values τ(x,y) and τ(x′,y′) may be used toprovide a first equation for the slope of the piecewise linearapproximation of the thickness. Similarly, points (x,y) and(x_(m),y_(n)) and their thickness values τ(x,y) and τ(x_(m),y_(n)) maybe used to provide a second equation for the slope of the piecewiselinear approximation of the thickness. The two slope equations may beequated and solved for τ(x′,y′). An approximation to the form factor ofequation [25] follows from [26]:

$\begin{matrix}\begin{matrix}{{{Aff}\left( {x_{m},y_{n},x,y} \right)} \approx \left\{ {1 + {\gamma \cdot \left\lbrack {{\frac{1}{M} \cdot {\tau\left( {x,y} \right)}} - {\frac{1}{M} \cdot {\tau\left( {x_{m},y_{n}} \right)}}} \right\rbrack}} \right\}} \\{{= \left\{ {1 + {\frac{\gamma}{M} \cdot \left\lbrack {{\tau\left( {x,y} \right)} - {\tau\left( {x_{m},y_{n}} \right)}} \right\rbrack}} \right\}},}\end{matrix} & \lbrack 27\rbrack\end{matrix}$which is a simple function of τ(x,y) and τ(x_(m),y_(n)). We may alsomultiply Aff(*) or its approximations with the symmetric kernel form ofequation [8] to obtain a curve that has excellent fit to the profilefrom the Monte Carlo simulation of the wedge object case (the curve withtriangle indicators shown in FIG. 15). The corresponding approximationfor equation [25A] is:

$\begin{matrix}{{{Aff}^{\;{Ibp}}\left( {x_{m},y_{n},x,y} \right)} \approx \left\{ {1 + {\frac{\gamma}{\mu \cdot M} \cdot \left\lbrack {{\ln\left( {I_{bp}\left( {x_{m},y_{n}} \right)} \right)} - {\ln\left( {I_{bp}\left( {x,y} \right)} \right)}} \right\rbrack}} \right\}} & \left\lbrack {27A} \right\rbrack\end{matrix}$While the above forms of Aff(*), Aff^(Ibp)(*), and their approximationshave been illustrated with the wedge-shaped object, they work very wellfor oval-shaped objects, such as human torsos and limbs. From equations[27] and [27A], the following more general asymmetric form factors canbe identified as:

$\begin{matrix}{{{Aff}^{\;{G\; 1}}\left( {x_{m},y_{n},x,y} \right)} = \left\{ {1 + {\sum\limits_{j = 1}^{J}\left( {{a_{j} \cdot {\tau^{j}\left( {x,y} \right)}} + {b_{j} \cdot {\tau^{j}\left( {x_{m},y_{n}} \right)}}} \right)}} \right\}} & \left\lbrack {27B} \right\rbrack \\{{{Aff}^{\;{G\; 2}}\left( {x_{m},y_{n},x,y} \right)} = \left\{ {1 + {\sum\limits_{j = 1}^{J}\left( {{c_{j} \cdot \left\lbrack {\ln\left( {I_{bp}\left( {x_{m},y_{n}} \right)} \right)} \right\rbrack^{j}} + {d_{j} \cdot \left\lbrack {\ln\left( {I_{bp}\left( {x,y} \right)} \right)} \right\rbrack^{j}}} \right)}} \right\}} & \left\lbrack {27C} \right\rbrack \\{\mspace{79mu}{{{Aff}^{\;{G\; 3}}\left( {x_{m},y_{n},x,y} \right)} = \left\{ {1 + {f_{1}\left( {{\tau\left( {x,y} \right)},{\tau\left( {x_{m},y_{n}} \right)}} \right)}} \right\}}} & \left\lbrack {27D} \right\rbrack \\{\mspace{79mu}{{{Aff}^{\;{G\; 3}}\left( {x_{m},y_{n},x,y} \right)} = \left\{ {1 + {f_{2}\left( {{I_{bp}\left( {x,y} \right)},{I_{bp}\left( {x_{m},y_{n}} \right)}} \right)}} \right\}}} & \left\lbrack {27E} \right\rbrack\end{matrix}$Where j is an index, J has a value of one or more, and a_(j), b_(j),c_(j) and d_(j) are constants that can have positive, negative, or zerovalues, or values that vary with location (e.g., coordinates x and y),and where ƒ₁(*) and ƒ₂(*) are generalized functions that may have anyform that includes their arguments. Corresponding generalized versionsof equations [25] and [25A] may be obtained by replacing τ(x, y) with

${\tau\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)},$and I_(bp)(x, y) with

${I_{bp}\left( {{\frac{x - x_{m}}{M} + x_{m}},{\frac{y - y_{n}}{M} + y_{n}}} \right)}.$

With the above in mind, a general class of asymmetric kernels may havethe following form:Ak ^(G)(x _(m) ,y _(n) ,x,y),  [28]where x_(m) and y_(n) are the location coordinates of the pencil beam,where x and y are the location coordinates of the scattered radiationmodeled by Ak^(G)(*), and where Ak^(G)(*) may include values of theobject thickness determined from the values of x_(m), y_(n), x, and y.The above discussion of equations [24]-[27] has touched on the followingsub-class of the general class:Ak ^(SC1)(x _(m) ,y _(n) ,x−x _(m) ,y−y _(n))=Sk ^(G)(x _(m) ,y _(n),x−x _(m) ,y−y _(n))·Aff ^(G)(x _(m) ,y _(n) ,x,y),  [29]where Sk^(G)(x_(m), y_(n), x−x_(m), y−y_(n)) is a general symmetrickernel form, and Aff^(G)(*) is a general asymmetric form factor to theasymmetric form factor of equation [25]. Each of Sk^(G)(*) andAff^(G)(*) may include values of the object thickness or radiationintensity values determined from the values of x_(m), y_(n), x, and y. Afurther sub-class uses an approximate asymmetric form factor thatdepends on the difference of τ(x,y) and τ(x_(m),y_(n)):Ak ^(SC2)(x _(m) ,y _(n) ,x−x _(m) ,y−y _(n))=Sk ^(G)(x _(m) ,y _(n),x−x _(m) ,y−y _(n))·Aff ^(Aprox)(τ_(xy)−τ_(bp)),  [30]where τ_(xy) is shorthand for τ(x,y), and τ_(bp) is shorthand forτ(x_(m),y_(n)). The symmetric kernel form of equation [8] may becombined with the approximate asymmetric form factor of equation [27] asfollows:Ak ^(A)(x _(m) ,y _(n) ,x−x _(m) ,y−y _(n))=S _(A)(I ₀ ,I _(bp))·S_(FF)(x−x _(m) ,y−y _(n))·{1+γ′·(τ_(xy)−τ_(bp))}  [31]where γ′ may be equal to γ/M or be empirically determined.

For imaging devices 120 that have small dimensions relative to theirdistance to the object, the scatter profile I_(os) from the object maybe generated from any of the above asymmetric kernel forms, asrepresented by Ak^(X)(*), as follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {\sum\limits_{m = {- M}}^{+ M}{\sum\limits_{n = {- N}}^{+ N}{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {Ak}^{x}{{(*}{).}}}}}} & \lbrack 32\rbrack\end{matrix}$For the asymmetric kernel form of equation [31], this may be written as:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {\sum\limits_{m = {- M}}^{+ M}{\sum\limits_{n = {- N}}^{+ N}{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)} \cdot {S_{FF}\left( {{x - x_{m}},{y - y_{n}}} \right)} \cdot {\left\{ {1 + {\gamma^{\prime} \cdot \left( {\tau_{xy} - \tau_{bp}} \right)}} \right\}.}}}}} & \lbrack 33\rbrack\end{matrix}$As equation [33] is based from equation [8A], both I_(bp) and S_(A)(I₀,I_(bp)) are functions of x_(m) and y_(n). Equation [33] may be separatedinto three double summations according to the three terms 1, γ′·τ_(xy),and −γ′·τ_(bp) in the last quantity, which may be generated by threecorresponding convolutions, as follows:

I_(os)(x, y) = [I_(bp)(x, y) ⋅ S_(A)(I₀, I_(bp))] ⊗ S_(FF)(x, y) + γ^(′) ⋅ τ(x, y) ⋅ {[I_(bp)(x, y) ⋅ S_(A)(I₀, I_(bp))] ⊗ S_(FF)(x, y)} −   [γ^(′) ⋅ τ(x, y) ⋅ I_(bp)(x, y) ⋅ S_(A)(I₀, I_(bp))] ⊗ S_(FF)(x, y),where each of I_(bp)(x,y) and S_(A)(I₀, I_(bp)) are now functions of xand y, and where (x,y) replaces (x_(m),y_(n)) in accordance with thecommonly used convolution operation notation ({circle around (x)}). Theabove can be simplified by combining the first two terms as follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} - {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {{S_{FF}\left( {x,y} \right)}.}}}}} & \lbrack 34\rbrack\end{matrix}$Equation [34] may be viewed as the addition of two convolution products;the first is the convolution of the product[I_(bp)(x,y)·S_(A)(I₀,I_(bp))] with the symmetric form factorS_(FF)(x,y) post-weighted by (1+γ′·τ(x,y)), and the second is theproduct [I_(bp)(x,y)·S_(A)(I₀,I_(bp))] pre-weighted by γ′·τ(x,y) andthereafter convoluted with the symmetric form factor S_(FF)(x,y). Thefollowing Fourier transform formulation readily follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot {??}^{- 1}}\left\{ {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{FF}\left( {x,y} \right)} \right)}} \right\}} - {{??}^{- 1}\left\{ {{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{FF}\left( {x,y} \right)} \right)}} \right\}}}} & \lbrack 35\rbrack\end{matrix}$where ℑ{*} denote a Fourier transform operation, and ℑ⁻¹{*} denotes aninverse Fourier transform.

In equations [34] and [35], each of I₀, I_(bp), and S_(A)(I₀, I_(bp)) isa function of coordinates x and y (instead of the pencil beamcoordinates x_(m) and y_(n)). As before, the scattered radiation profileI_(os)(x,y) is relatively smooth (i.e., slowly varying) over the x,ydomain, and its Fourier transform has primarily low-frequencycomponents. Accordingly, the Fourier transforms in equation [11] can begenerated from a decimated set of data (e.g.,[I_(bp)(x,y)·S_(A)(I₀,I_(bp))]) on a coarse grid, such as the 104 by 30array of super pixels previously described. In practice, the summationof equation [33] and the convolutions of equation [34] are done over thefinite area of the pixel array, which means that contributions frompencil beams that terminate outside of the array are not reflected inI_(os)(x,y). This can affect the scatter values at the edges of thearray, and the previously-described ways for compensating for it withregard to the application of the symmetric kernels may be used here. Itshould be said that this is not a problem if the radiation shadow of theobject falls within the pixel array. In this case, there is noscattering contribution from pencil beams that terminate outside of thepixel array.

While we have used the approximation {1+γ′·(τ_(xy)−τ_(bp))} in equation[33], other approximations may be used, such as{1+γ₁·(τ_(xy)−τ_(bp))+γ₂·(τ_(xy)−τ_(bp))²} or{1+γ₃·(τ_(xy)−τ_(bp))+γ₄·(τ_(xy) ²−τ_(bp) ²)}. These forms, as well andmay more, are contemplated by these inventions of the presentapplication, and are covered by appropriate broad claims of theapplication.

Referring briefly back to the imaging chain of FIG. 4B, one use of theasymmetric kernel is to estimate the individual profiles I_(o0) andI_(os) from their sum (I_(o0)+I_(os)). As described above, it waspossible to generate an estimate of the radiation profile I_(I) at thefront of the detector housing from the measured data from imaging device120, where I_(I)=(I_(o0)+I_(os)). Equation [34] can be used to estimatethe profile I_(os), and we can write the following relationship:

$\begin{matrix}{{I_{I}\left( {x,y} \right)} = {{I_{o\; 0}\left( {x,y} \right)} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} - {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {{S_{FF}\left( {x,y} \right)}.}}}}} & \lbrack 36\rbrack\end{matrix}$In this use of the asymmetric kernel, the coordinates x_(n), y_(n), x,and y are known, and I₀ can be measured by a calibration step. However,τ(x,y) and I_(bp)(x,y) are not known, with I_(bp)(x,y) being the profileI_(o0) that is to be estimated from the I_(I)(x,y). In the applicationsof the symmetric kernels, an iterative approach was used to handle theunknown state of I_(bp)(x,y). For solving equation [36], a two-leveliteration approach may be used as follows. A first tomographicreconstruction is performed using uncorrected projections, orprojections corrected by the application of one or more symmetrickernels, to estimate the thickness values of the object for all theprojections. Then, the value of I_(o0)(x,y) (I_(bp)(x,y)) may beiterated in equation [36], using the estimated thickness values forτ(x,y), to generate estimates of scatter for each of the projections.Corrected projections may then be generated, and a second tomographicreconstruction may be preformed to provide more true results. Ifdesired, these steps may be reiterated one or more times to furtherrefine the corrections (e.g., a second round of thickness estimates canbe generated from the results of second tomographic reconstruction, asecond round of the corrected projections can then be generated from thesecond round of thickness estimates, and a third tomographicreconstruction can be generated from the second round of correctedprojections).

However, using the tomographic reconstructions in this manner iscomputationally expensive and time-consuming. As another approach, wenote that the object's thickness is substantially proportional to ln{I₀/I_(bp)(x,y)}, which can be used as a proxy for the object thickness.A general thickness function T_(I)(I₀, I_(bp)(x,y)) may be constructedfrom measured empirical data; it may have a form as simple as T_(I)(I₀,I_(bp))=ln {I₀/I_(bp)(x,y)}/μ, where μ is around 0.19 to 0.21 cm⁻¹ forwater- and soft-tissue like materials (density of ˜1 g/cc) irradiated byradiation with typical beam energies. The use of the log-ratio ofintensity values captures not only thickness variations, but alsodensity variations in the object, which also can have asymmetric effectson the scattering. Accordingly, the use of T_(I)(I₀, I_(bp)) to providethe thickness values for τ(x,y) is currently believed to provide a moregeneral modeling of the asymmetric effects. In addition, the use ofT_(I)(I₀, I_(bp)) makes the term γ′·τ(x,y) independent of μ, thereforemakes for an easier implementation. With this approach, a single leveliteration approach may be constructed as follows. An initial estimate I⁰_(o0)(x,y) may be generated for I_(o0)(x,y). The initial estimate I⁰_(o0)(x,y) may be a fraction of I_(I)(x,y), such as I_(I)(x,y)/2, or theestimate for I_(o0)(x,y) generated for a prior projection in the objectscan. From the initial estimate I⁰ _(o0)(x,y), an estimated initialvalue I¹ _(EST)(x,y) for I_(bp)(x,y) may be generated (I¹ _(EST)(x,y)=I⁰_(o0)(x,y)), and an initial value S_(A) ¹(I₀,I_(EST)) for the amplitudefunction S_(A)(I₀,I_(bp)) may be generated. Also, an initial valueτ¹(x,y) for τ(x,y) may be estimated from the thickness function asτ¹(x,y)=T_(I)(I₀, I¹ _(EST)(x,y)). A first iteration estimate I¹_(o0)(x,y) may be generated for I_(o0)(x,y) by applying equation [36] asfollows:

$\begin{matrix}{{I_{o\; 0}^{1}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - {\left( {1 + {\gamma^{\prime} \cdot {\tau^{1}\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} + {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau^{1}\left( {x,y} \right)} \cdot {I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}}}}} & \lbrack 37\rbrack\end{matrix}$where the convolutions may be performed using Fourier transforms asdescribed above. More refined estimates of I_(EST)(x,y) and S_(A)(I₀,I_(EST)) may then be generated from I¹ _(o0)(x,y), and equation [37] maybe reiterated to further refine the estimate for I_(o0)(x,y). Thefollowing iteration approach may be used for subsequent iterations k=2to K for generating refined estimates I^(k) _(o0)(x,y):

$\begin{matrix}{\mspace{79mu}{{I_{EST}^{k}\left( {x,y} \right)} = {{0.5*{I_{o\; 0}^{k - 1}\left( {x,y} \right)}} + {0.5*{I_{o\; 0}^{k - 2}\left( {x,y} \right)}}}}} & \left\lbrack {38A} \right\rbrack \\{\mspace{79mu}{{\tau^{k}\left( {x,y} \right)} = {T_{I}\left( {I_{0},{I_{EST}^{k}\left( {x,y} \right)}} \right)}}} & \left\lbrack {38B} \right\rbrack \\{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - {\left( {1 + {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} + {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {{S_{FF}\left( {x,y} \right)}.}}}}} & \left\lbrack {38C} \right\rbrack\end{matrix}$Equation [38A] provides some damping on the rate of change of I^(k)_(EST)(x,y), S_(A) ^(k)(I₀,I^(k) _(EST)), and τ^(k)(x,y) between theiterations compared to the selection of I^(k) _(EST)(x,y)=I^(k-1)_(o0)(x,y), and minimizes the chances of the values of I^(k)_(EST)(x,y), S_(A) ^(k)(I₀, I_(EST)), and τ^(k)(x,y) oscillating duringthe iterations. An equal weighting factor of 0.5 has been used inequation [38A], but different weighting factors may be used (at leastthe first weighting factor should be non-zero and both weighting factorsshould sum to 1). Equations [37] and [38] may be applied to the realpixel array, or to the super pixel array. In the latter case, the superpixel values for I^(k) _(o0)(x,y) and I_(I)(x,y) may be generated asaverages in the manner described above for the quantity[I_(bp)(x,y)·S_(A)(I₀, I_(bp))], and, if the Fourier transform processhas been used to generate the convolution terms with decimated data, theresults of the inverse Fourier transforms do not need to beinterpolated/extrapolated. The iterations may be continued until thechange between iteration values of [I^(k) _(EST)(x,y)·S_(A)^(k)(I₀,I^(k) _(EST))] and [I^(k-1) _(EST)(x,y)·S_(A) ^(k-1)(I₀,I^(k-1)_(EST))] is below a desired (e.g., set) amount for a desired (e.g., set)number of pixels or super pixels, or until the change between iterationvalues of I^(k) _(o0)(x,y) and I^(k-1) _(o0)(x,y) is below a desiredamount for a desired number of pixels or super pixels. Additionalcriteria or other criteria may be used to determine when to stop theiterations, such as monitoring an average measure of the changes overthe pixels or super pixels, and setting an upper limit to the number ofiterations. If the iterations have been carried out on the super pixels,then one application of equation [38C] using the real pixel array may beused to generate I^(k) _(o0)(x,y) values over the real pixel array afterthe last iteration (k=K). For this, the convolution term may begenerated on the super pixel array and then interpolated andextrapolated onto the real pixel array. The scattered radiation at anyiteration, including the last iteration k=K, may be generated as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = {{{+ \left( {1 + {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)}}} \right)} \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} - {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}}}}} & \left\lbrack {38D\text{-}1} \right\rbrack\end{matrix}$or as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = {{{+ \left( {1 + {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)}}} \right)} \cdot \left\{ {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} - {\quad{\left\lbrack {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)} \cdot {I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {{S_{FF}\left( {x,y} \right)}.}}}}} & \left\lbrack {38D\text{-}2} \right\rbrack\end{matrix}$

Another exemplary iteration approach similar to equation [15] forsubsequent iterations k=2 to K is as follows:

$\begin{matrix}{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5 \cdot \left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5 \cdot \left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {39A} \right\rbrack\end{matrix}$

Generate τ^(k)(x,y) from prior reconstruction, or as T_(I)(I₀, I^(k-1)_(o0)(x,y)),or as 0.5·T _(I)(I ₀ ,I ^(k-1) _(o0)(x,y))+0.5·τ^(k-1)(x,y)  [39B]

$\begin{matrix}{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - {\left( {1 + {\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)}}} \right) \cdot \left\{ {\begin{bmatrix}{{I_{EST}^{k}\left( {x,y} \right)} \cdot} \\{S_{A}\left( {I_{0},I_{EST}^{k}} \right)}\end{bmatrix} \otimes {S_{FF}\left( {x,y} \right)}} \right\}} + {\begin{bmatrix}\begin{matrix}{\gamma^{\prime} \cdot {\tau^{k}\left( {x,y} \right)} \cdot} \\{{I_{EST}^{k}\left( {x,y} \right)} \cdot}\end{matrix} \\{S_{A}\left( {I_{0},I_{EST}^{k}} \right)}\end{bmatrix} \otimes {S_{FF}\left( {x,y} \right)}}}} & \left\lbrack {39C} \right\rbrack\end{matrix}$with [I¹ _(EST)(x,y)S_(A) ¹(I₀,I¹ _(EST))]=[I⁰ _(o0)(x,y)·S_(A)(I₀, I⁰_(o0))]. Equation [39A] generates a scatter estimate for the currentiteration k as a weighted average of the scatter as generated from themost recent estimate of I₀ (which is I^(k-1) _(o0)(x,y)) and the scatterused in the last iteration. Equation [39B] can generate τ^(k)(x,y) forthe current iteration k as a weighted average of the thickness generatedfrom the most recent estimate of I_(o0) (which is I^(k-1) _(o0)(x,y))and τ^(k-1)(x,y) used in the last iteration. Like Equation [38A],equation [39A] provides some damping, and minimizes the chances of thevalues oscillating during the iterations. An equal weighting factor of0.5 has been used in equation [39A], but different weighting factors maybe used (at least the first weighting factor should be non-zero and bothweighting factors should sum to 1). Equations [37] and [39] may beapplied to the real pixel array, or to the super pixel array. In thelatter case, the super pixel values for I^(k) _(o0)(x,y) and I_(I)(x,y)may be generated as averages in the manner described above for thequantity [I_(bp)(x,y)·S_(A)(I₀, I_(bp))], and, if the Fourier transformprocess has been used to generate the convolution terms with decimateddata, the results of the inverse Fourier transforms do not need to beinterpolated/extrapolated. The above possible criteria for determiningwhen to end the iterations of equation [38] may be used to determinewhen to end the iterations of equation [39]. If the iterations have beencarried out on the super pixels, then one application of equation [39C]using the real pixel array may be used to generate I^(k) _(o0)(x,y)values over the real pixel array. For this, the convolution term may begenerated on the super pixel array and then interpolated andextrapolated onto the real pixel array. The scattered radiation at anyiteration, including the last iteration k=K, may be generated usingequations [38D].

In equations [37]-[39] values for τ(x,y) may be provided by theabove-described thickness function T_(I)(*), by a prior tomographicreconstruction of the object, or by an estimated shape of the object. Inequations [37], [38C] and [39C], the convolutions are preferablygenerated by the Fourier transform method, but can be generated by afull-space convolution using equation [34], or by a full-space summationusing equation [33], or by a full-space summation using equation [33]with the form of equation [25] in place of {1+γ′·(τ_(xy)−τ_(bp))}. Theseimplementations, though slow, would allow use of the full skewcorrection factor Z(θ_(p)), or in the last case avoid the piecewiseapproximation used in equation [26]. Skew correction factors Z′(θ_(p))may be applied to each of equations [37], [38C] and [39C] by multiplyingit with each of the amplitude functions, as was done with equation[10A]. The application of skew correction factors Z″(*) to each ofequations [37], [38C] and [39C] follows a similar form to that ofequation [10B], but leads to the summation of six convolution terms,which can be readily handled by the Fourier transform form of equation[35]. This form for I_(os)(x,y) is provided below:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{\cos^{3}\left( \theta_{p} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} \right\}} - {\quad{{\left\lbrack {{\cos^{3}\left( \theta_{p} \right)} \cdot \gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{FF}\left( {x,y} \right)}} - {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ \left\lbrack {{2 \cdot {\cos^{3}\left( \theta_{p} \right)} \cdot {\tan\left( {\left. \quad\theta_{x} \right) \cdot {\left. \quad{{I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes \left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {x/d} \right)} \right\rbrack}} \right\}}} + {\quad\left\lbrack {{2 \cdot {\cos^{3}\left( \theta_{p} \right)} \cdot {\tan\left( \theta_{x} \right)} \cdot \gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {\left. \quad{S_{A}\left( {I_{0},I_{bp}} \right)} \right\rbrack \otimes \left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {x/d} \right)} \right\rbrack}} - {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {2 \cdot {\cos^{3}\left( \theta_{p} \right)} \cdot {\tan\left( \theta_{y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes \left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {x/d} \right)} \right\rbrack} \right\}} + {\quad{\left\lbrack {2 \cdot {\cos^{3}\left( \theta_{p} \right)} \cdot {\tan\left( \theta_{x} \right)} \cdot \gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {\quad{\left\lbrack {{S_{FF}\left( {x,y} \right)} \cdot \left( {x/d} \right)} \right\rbrack.}}}}} \right.}} \right. \right.}}}}} & \lbrack 40\rbrack\end{matrix}$

Multiple Asymmetric Kernel Forms for Different Thickness Ranges.

As indicated above, another invention set of the present applicationrelates to constructing two or more instances of the new asymmetric formfor two or more respective ranges of object thickness, and applying theinstances of the new symmetric form to regions of a radiographicprojection according to a measure of the object's thickness in theregions. An exemplary embodiment of the invention is illustrated withthree instances Ak₁(*), Ak₂(*), and Ak₃(*) of an asymmetric kernel thatcomprise the previously described kernel instances Sk₁(*), Sk₂(*), andSk₃(*), respectively, multiplied by an asymmetric form factor Aff(*).Sk₁(*), Sk₂(*), and Sk₃(*) were previously described with respect toFIGS. 8-10. The corresponding thickness functions T₁(*), T₂(*), andT₃(*) of Sk₁(*), Sk₂(*), and Sk₃(*) can be used to determine the valuesof primary beam radiation intensity (I_(bp)) that separate the thicknessregions from one another. As before, it can be found from the thicknessfunctions that an intensity value I_(A) separates the first thicknessregion (0-10 cm, modeled by kernels Sk₁(*) and Ak₁(*)) from the secondthickness region (10-25 cm, modeled by kernels Sk₂(*) and Ak₂(*)), andthat an intensity value I_(B) separates the second thickness region fromthe third thickness region (25 cm and above, modeled by kernels Sk₃(*)and Ak₃(*)), with I_(A) being greater than I_(B). During each iterationof either of equations [38] or [39], estimated primary beam profileI^(k) _(EST)(x,y) can be determined from either the initial value or theresults of the previous iteration. Each pixel value of I^(k) _(EST)(x,y)may be compared with I_(A) and I_(B) to determine which kernel should beused to generate the scattered radiation associated with the pencil beamterminating at the pixel. For pixels or super pixels having I^(k)_(EST)(x,y) greater than I_(A), the first asymmetric kernel instanceAk₁(*) may be used. For pixels or super pixels having I^(k) _(EST)(x,y)equal to or less than I_(A) and greater than I_(B), the secondasymmetric kernel instance Ak₂(*) may be used. For pixels or superpixels having I^(k) _(EST)(x,y) equal to or less than I_(B), the thirdasymmetric kernel instance Ak₃(*) may be used.

The summation-based equation [32] for I_(os)(x,y) may be expanded toinclude three separate double summations, one for each of the kernelinstances: Ak₁(*), Ak₂(*), and Ak₃(*). Each double summation may be onlyover the group of pixels covered by the kernel of the double summation.The expanded summation-based equation is as follows:

$\begin{matrix}{{I_{ox}\left( {x,y} \right)} = {{{\sum\limits_{{Group}\mspace{14mu} 1}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Ak}_{1}\left( \,^{*} \right)}}}} + {\sum\limits_{{Group}\mspace{14mu} 2}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Ak}_{2}\left( \,^{*} \right)}}}} + {\sum\limits_{{Group}\mspace{14mu} 3}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Ak}_{3}\left( \,^{*} \right)}}}}} = {{\sum\limits_{{Group}\mspace{14mu} 1}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Sk}_{1}\left( {x_{m},y_{n},{x - x_{m}},{y - y_{n}}} \right)} \cdot {{Aff}\left( {x_{m},y_{n},x,y} \right)}}}} + {\sum\limits_{{Group}\mspace{14mu} 2}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Sk}_{2}\left( {x_{m},y_{n},{x - x_{m}},{y - y_{n}}} \right)} \cdot {{Aff}\left( {x_{m},y_{n},x,y} \right)}}}} + {\sum\limits_{{Group}\mspace{14mu} 3}{\sum{{I_{bp}\left( {x_{m},y_{n}} \right)} \cdot {{Sk}_{3}\left( {x_{m},y_{n},{x - x_{m}},{y - y_{n}}} \right)} \cdot {{{Aff}\left( {x_{m},y_{n},x,y} \right)}.}}}}}}} & \lbrack 41\rbrack\end{matrix}$As equation [41] is based from equations [32] and [8A], both I_(bp) andS_(A)(I₀, I_(bp)) are functions of x_(m) and y_(n). Any of the skewcorrection factors Z(*), Z′(*), or Z″(*) may be applied to equation [41]by multiplying it with each of the kernel instances. To obtain thecorresponding convolution form for I_(os)(x,y), the asymmetric formfactor may be approximated by equation [27], each symmetric kernelinstance may be written as the product of its amplitude function and itsform factor, e.g., Sk₁(x−x_(m), y−y_(n),I_(bp))=S_(A,1)(I₀,I_(bp))·S_(FF,1)(x,y), and each amplitude functionmay be set to a value of zero for pixels that are not covered by itskernel instance. For example, amplitude function S_(A,1)(I₀,I_(bp)) forSk₁(*) has zero values for the pixel locations in Groups 2 and 3, andnon-zero values for the pixels in Group 1; amplitude functionS_(A,2)(I₀,I_(bp)) for Sk₂(*) has zero values for the pixel locations inGroups 1 and 3, and non-zero values for the pixels in Group 2, etc. Withthat format of the amplitude functions and the form of Aff(*) providedby equation [25], and with the convolution solution provided by equation[34], the convolution form for an exemplary multiple kernelimplementation is:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\quad{{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\quad{{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\quad{{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}},}}}}}}}} & \lbrack 42\rbrack\end{matrix}$where each of I_(bp), S_(A,1)(I₀, I_(bp)), S_(A,2)(I₀, I_(bp)), andS_(A,3)(I₀, I_(bp)) are now functions of x and y for the convolutionform, and where values for τ(x,y) may be provided by the above-describedthickness function T_(I)(*), by a prior tomographic reconstruction ofthe object, or by an estimated shape of the object. If there are nopixels or super pixels in the third group, then the last two convolutionquantities may be omitted from equation [42] and other similar equationsprovided below. Skew correction factors Z′(θ_(p)) may be applied toequation [42] by multiplying it with each of the amplitude functions, aswas done with equation [10A]. The application of skew correction factorsZ″(*) to equation [42] follows a similar form to that of equation [40],but leads to the summation of 18 convolution terms.

Since the Fourier transform of two convolved functions is equal to themultiplication of their Fourier transforms, the profile I_(os) fromequation [42] can be generated from the Fourier transforms as follows:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot {??}^{- 1}}\left\{ {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},1}\left( {x,y} \right)} \right)}} \right\}} - {{??}^{- 1}\left\{ {{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},1}\left( {x,y} \right)} \right)}} \right\}} + {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot {??}^{- 1}}\left\{ {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},2}\left( {x,y} \right)} \right)}} \right\}} - {{??}^{- 1}\left\{ {{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},2}\left( {x,y} \right)} \right)}} \right\}} + {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot {??}^{- 1}}\left\{ {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},3}\left( {x,y} \right)} \right)}} \right\}} - {{??}^{- 1}\left\{ {{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},3}\left( {x,y} \right)} \right)}} \right\}}}} & \lbrack 43\rbrack\end{matrix}$which can be simplified as follows to reduce the number of inverseFourier transforms from six to two:

$\begin{matrix}{{I_{os}\left( {x,y} \right)} = {{{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot {??}^{- 1}}\left\{ {{{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},1}\left( {x,y} \right)} \right)}} + {{??}{\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right) \cdot {??}}\left( {S_{{FF},2}\left( {x,y} \right)} \right)} + {{{??}\left( {{I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},3}\left( {x,y} \right)} \right)}}} \right\}} - {{??}^{- 1}{\left\{ {{{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},1}\left( {x,y} \right)} \right)}} + {{??}{\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{bp}} \right)}} \right) \cdot {??}}\left( {S_{{FF},2}\left( {x,y} \right)} \right)} + {{{??}\left( {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{bp}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{bp}} \right)}} \right)} \cdot {{??}\left( {S_{{FF},3}\left( {x,y} \right)} \right)}}} \right\}.}}}} & \left\lbrack {43A} \right\rbrack\end{matrix}$Equations [43] are typically applied on the array of super pixels ratherthan the array of real pixels, using the same super pixel constructionprocesses as described above with regard to equations [11] and [35]. Inthis regard, an average value of I_(bp) over the pixels covered by asuper pixel may be used to determine which kernel instance to use forthe super pixel. The Fourier transform of the form factors ℑ{S_(FF,1)(x,y)}, ℑ{S_(FF,2)(x, y)}, and ℑ{S_(FF,3)(x, y)} may be generated by usingthe values of the form factors at the grid locations of the superpixels; they may be generated once, and may be used for all ofprojections of the scan. The inverse Fourier transforms of equations[43] and the results of equations [43] may be computed for the gridlocations of the super pixels, and the results of the inverse Fouriertransforms may be interpolated/extrapolated onto the real pixel array inany of the above-described manners. The Fourier transforms and inverseFourier transforms used herein may be implemented by any of the knowndiscrete Fourier transform processes. If there are no pixels or superpixels in the third group, then the Fourier quantities for it inequations [43] may be omitted from the equation, and other similarequations provided below.

Using equation [42], the corresponding form of equation [12A] forgenerating an estimate of I_(o0)(x,y) from I_(I)(x,y) is as follows:

$\begin{matrix}{{I_{o\; 0}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\{ {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {{\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes \left. \quad{S_{{FF},1}\left( {x,y} \right)} \right\}} - {\quad\left\lbrack {\left. \quad{{\left. \quad{\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{EST}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\}.} \right.}} \right.} \right.}} & \lbrack 44\rbrack\end{matrix}$An iteration approach similar to equations [38] and [39] may used togenerate and estimate of I_(o0)(x,y). An initial estimate for I⁰_(o0)(x,y) may be generated for I⁰ _(o0)(x,y), such as using a fractionof I_(I)(x,y). Each pixel value of the initial estimate I⁰ _(o0)(x,y)may be compared with the intensity values I_(A) and I_(B) to determinewhich kernel instance the pixel should be assigned to, as describedabove. Then, each kernel instance's amplitude functionS_(A,1)(I₀,I_(EST)), S_(A,2)(I₀,I_(EST)), and S_(A,3)(I₀,I_(EST)) isgenerated using the pixel values of I⁰ _(o0)(x,y) of the pixels thathave been assigned to the kernel instance. Next, a first iterationestimate I¹ _(o0)(x,y) may be generated for I_(o0)(x,y) by applyingequation [44] as follows:

$\begin{matrix}{{I_{o\; 0}^{1}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\lbrack {\quad{{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,1}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\quad{{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,1}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,2}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - \left. \quad{{\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,2}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{1}\left( {x,y} \right)} \cdot {S_{A,3}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{1}\left( {x, y} \right)} \cdot {S_{A,3}^{1}\left( {I_{0},I_{EST}^{1}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\rbrack}}}} \right.}} & \lbrack 45\rbrack\end{matrix}$where the convolution may be performed using Fourier transforms asdescribed above with reference to equations [43]. More refined estimatesof I_(EST)(x,y), τ(x,y), S_(A,1)(I₀, I_(EST)), S_(A,2)(I₀,I_(EST)), andS_(A,3)(I₀, I_(EST)) may then be generated from I¹ _(o0)(x,y), andequation [45] may be reiterated to further refine the estimate forI_(o0)(x,y). During this refinement, it is possible for there to bechanges in the kernel assignments for some of the pixels (or superpixels). The following iteration approach similar to equation [14] maybe used for subsequent iterations k=2 to K for generating refinedestimates I^(k) _(o0)(x,y):

$\begin{matrix}{\mspace{79mu}{{{I_{EST}^{k}\left( {x,y} \right)} = {{0.5*{I_{o\; 0}^{k - 1}\left( {x,y} \right)}} + {0.5*{I_{o\; 0}^{k - 2}\left( {x,y} \right)}}}},}} & \left\lbrack {46A} \right\rbrack \\{{{Generate}\mspace{14mu}{\tau^{k}\left( {x,y} \right)}},{S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)},{S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)},{{and}\mspace{14mu}{S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}},} & \left\lbrack {46B} \right\rbrack \\{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - {\left\lbrack {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\rbrack.}}} & \left\lbrack {46C} \right\rbrack\end{matrix}$Equation [46A] provides some damping on the rate of change of I^(k)_(EST)(x,y), the thickness function τ^(k)(x,y), and the amplitudefunctions S_(A,1) ^(k)(I₀,I^(k) _(EST)), S_(A,2) ^(k)(I₀,I^(k) _(EST)),and S_(A,3) ^(k)(I₀,I^(k) _(EST)) between the iterations, and minimizesthe chances of the function values oscillating during the iterations. Anequal weighting factor of 0.5 has been used in equation [46A], butdifferent weighting factors may be used (at least the first weightingfactor should be non-zero and both weighting factors should sum to 1).Also, the kernel-instance assignment of the pixels (or super pixels) maybe frozen for the latter iterations to prevent oscillations caused byswitching assignments at group boundaries.

Equations [45] and [46] may be applied to the real pixel array, or tothe super pixel array. In the latter case, the super pixel values forI^(k) _(o0)(x,y) and I_(I)(x,y) may be generated as averages in themanner described above for equations [13]-[15], and, if the Fouriertransform process (equation [43]) has been used to generate theconvolution terms from decimated data, the results of the inverseFourier transforms do not need to be interpolated/extrapolated duringthe iterations. The iterations may be continued until the change betweenthe current and previous values of each amplitude function, asmultiplied by I^(k) _(EST)(x,y), is below a desired (e.g., set) amountfor a desired (e.g., set) number of pixels or super pixels, or until thechange between iteration values of I^(k) _(o0)(x,y) and I^(k-1)_(o0)(x,y) is below a desired amount for a desired number of pixels orsuper pixels. Additional criteria or other criteria may be used todetermine when to stop the iterations, as described above for equations[13]-[15]. If the iterations have been carried out on the super pixels,then one application of equation [46C] using the real pixel array may beused to generate I^(k) _(o0)(x,y) values over the real pixel array afterthe last iteration (k=K). For this, the convolution term may begenerated on the super pixel array and then interpolated andextrapolated onto the real pixel array. The scattered radiation at anyiteration, including the last iteration k=K, may be generated as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = \left\lbrack {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \;\left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\rbrack} & \left\lbrack {46D\text{-}1} \right\rbrack\end{matrix}$or as:

$\begin{matrix}{{I_{os}^{k}\left( {x,y} \right)} = {\left\lbrack {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{o\; 0}^{k}\left( {x,y} \right)} \cdot {S_{A\;,3}^{k}\left( {I_{0},I_{o\; 0}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\rbrack.}} & \left\lbrack {46D\text{-}2} \right\rbrack\end{matrix}$

Another iteration approach similar to that of equation [15] forsubsequent iterations k=2 to K is as follows:

$\begin{matrix}{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,1}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,1}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {47A\text{-}1} \right\rbrack \\{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,2}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,2}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {47A\text{-}2} \right\rbrack \\{\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack = {{0.5*\left\lbrack {{I_{o\; 0}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,3}\left( {I_{0},I_{o\; 0}^{k - 1}} \right)}} \right\rbrack} + {0.5*\left\lbrack {{I_{EST}^{k - 1}\left( {x,y} \right)} \cdot {S_{A,3}^{k - 1}\left( {I_{0},I_{EST}^{k - 1}} \right)}} \right\rbrack}}} & \left\lbrack {47A\text{-}3} \right\rbrack\end{matrix}$

Generate τ^(k)(x,y) from prior reconstruction, or as T_(I)(I₀, I^(k-1)_(o0)(x,y)),or as 0.5·T _(I)(I ₀ ,I ^(k-1) _(o0)(x,y))+0.5·τ^(k-1)(x,y)  [47B]

$\begin{matrix}{{I_{o\; 0}^{k}\left( {x,y} \right)} = {{I_{I}\left( {x,y} \right)} - \left\lbrack {{\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,1}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},1}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,2}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},2}\left( {x,y} \right)}} + {\left( {1 + {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)}}} \right) \cdot \left\{ {\left\lbrack {{I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}} \right\}} - {\left\lbrack {\gamma^{\prime} \cdot {\tau\left( {x,y} \right)} \cdot {I_{EST}^{k}\left( {x,y} \right)} \cdot {S_{A,3}^{k}\left( {I_{0},I_{EST}^{k}} \right)}} \right\rbrack \otimes {S_{{FF},3}\left( {x,y} \right)}}} \right\rbrack}} & \left\lbrack {47C} \right\rbrack\end{matrix}$with [I¹ _(EST)(x,y)*S¹ _(A,1)(I₀,I¹ _(EST))]=[I⁰_(o0)(x,y)·S_(A,1)(I₀,I⁰ _(o0))], [I¹ _(EST)(x,y)·S¹ _(A,2)(I₀,I¹_(EST))]=[I⁰ _(o0)(x,y)·S_(A,2)(I₀,I⁰ _(o0))], and [I¹ _(EST)(x,y)·S¹_(A,3)(I₀,I¹ _(EST))]=[I⁰ _(o0)(x,y)·S_(A,3)(I₀,I⁰ _(o0))]. LikeEquation [46A], equations [47A] provide some damping on the rate ofchange of the thickness and amplitude functions between the iterations,and minimizes the chances of the function values oscillating during theiterations. An equal weighting factor of 0.5 has been used in equation[47A], but different weighting factors may be used (at least the firstweighting factor should be non-zero and both weighting factors shouldsum to 1). If the kernel assignment of a pixel or super pixel changesduring an iteration, its I_(EST)(*)·S_(A,X)(*) value will be averagedwith zero values for subsequent iterations if S_(A,X)(*) changes from anon-zero value to a zero value, and its I_(EST)(*)S_(A,X)(*) value willstart from zero and be averaged with non-zero value for subsequentiterations if S_(A,X)(*) changes from a zero value to a non-zero value,thereby providing a smooth transition. Equations [45]-[27] may beapplied to the real pixel array, or to the super pixel array. In thelatter case, the super pixel values for I^(k) _(o0)(x,y) and I_(I)(x,y)may be generated as averages in the manner described above for equations[13]-[15], and, if the Fourier transform process (equation [43]) hasbeen used to generate the convolution terms from decimated data, theresults of the inverse Fourier transforms do not need to beinterpolated/extrapolated during the iterations. The above possiblecriteria for determining when to end the iterations of equation [46] maybe used to determine when to end the iterations of equation [47]. If theiterations have been carried out on the super pixels, then oneapplication of equation [47C] using the real pixel array may be used togenerate I^(k) _(o0)(x,y) values over the real pixel array. For this,the convolution term may be generated on the super pixel array and theninterpolated and extrapolated onto the real pixel array. The scatteredradiation at any iteration, including the last iteration k=K, may begenerated using equations [46D].

In equations [45]-[47] values for τ(x,y) may be provided by theabove-described thickness function T_(I)(*), by a prior tomographicreconstruction of the object, or by an estimated shape of the object. Inequations [46C] and [47C], the convolution terms are preferablygenerated by the Fourier transform method (equation [43]), but they canbe generated by a full-space convolution using equation [42], or by afull-space summation using equation [41], or by a full-space summationusing equation [41] with the form of equation [25] in place of{1+γ′·(τ_(xy)−τ_(bp))}. These implementations, though slow, would allowuse of the full skew correction factor Z(θ_(p)). Skew correction factorsZ′(θ_(p)) may be applied to each of equations [45], [46C] and [47C] bymultiplying it with each of the amplitude functions, as was done withequation [10A]. The application of skew correction factors Z″(*) to eachof equations [45], [46C] and [47C] follows a similar form to that ofequation [40], but leads to the summation of 18 convolution terms, butwhich can be readily handled by the Fourier transform form of equation[43].

Edge-Effect Compensation.

Monte Carlo simulations of the kernel for a uniform-slab object showthat, towards the edge of an object, higher-order scattering events arereduced due to the absence of scattering media. This reduces theamplitude of the scatter kernel at the object boundary by approximately15%. These simulations are shown on a relative scale in FIG. 16 forvarious distances from the edge (0.1 cm, 1 cm, 2 cm, 5 cm, 10 cm, 30cm), with the edge being located to the left of the peaks, and theobject extending from the edge to the right of the graph. The slab has auniform thickness of 15 cm. FIG. 17 shows the kernel amplitude change asa function of the distance of the incident beam from the edge (solidline curve). At the very edge, the kernel amplitude is reduced by 15%;while at 5 cm from the edge, the amplitude is only reduced by 4%. Theamplitude change can be approximated by the straight dashed line, whichstarts at a value of 0.85 at the edge and reaches a value of 1 at adistance of 7 cm within the object. The amplitude change can also beapproximated by the dashed polynomial line that reaches a value of 1 ata distance of 12 cm within the object.

Several approaches may be used to account for the edge effect whengenerating estimates of I_(os)(x,y), such as by application of any ofequations [9]-[12], [10A], [10B], [11A], [17]-[20], [32]-[36],[40]-[44], or when using estimates of I_(os)(x,y) in the equations forthe iterative solutions [13]-[15], [21]-[23], [37]-[39], and [45]-[47].A first exemplary approach is to identify the edge of the object in thepixel array or the super pixel array, to generate a kernel modificationfunction Mk(x,y) that spans the array and accounts for the edge effect,and to multiply the kernel by Mk(x,y) in the above equations, such as bymultiplying it with the kernel's amplitude function (e.g., S_(A)(*)).The kernel modification function Mk(x,y) has a value of zero for pixelsand super pixels that are outside of the radiation shadow of the object,a value of ˜0.85 at the edge, values between ˜0.85 and 1 inside theobject near the edge, and values of 1 at the remaining interior pixelsand super pixels of the object. For typical human objects, the objectedges are generally parallel to the y-axis. Each pixel in the aboveexemplary 1024×768 pixel array measures 0.39 mm by 0.39 mm, and each ofthe above exemplary 9×25 or 10×26 super pixels for the exemplary arraymeasures 3.5 mm by 9.75 mm. Thus, for a 7 cm to 12 cm span of the edgeeffect along a y-axis row of pixels or super pixels, the edge effectcovers approximately 180 to 308 pixels, and approximately 20 to 34 superpixels.

A first exemplary method of generating the kernel modification functionMk(x,y) is as follows.

-   -   Action #1. For each pixel or super pixel, set the value of the        array element Mk(x,y) corresponding to a pixel or super pixel to        1 if the value of the measured radiation profile I_(I)(x,y) at        the pixel or super pixel is below a threshold value I_(TH);        otherwise set the value to zero. The threshold value I_(TH) may        be at or slightly below the value of I₀. Values of I_(I)(x,y)        above I_(TH) indicate the absence of the object, while values        below I_(TH) indicate the presence of the object.    -   Action #2. From the left edge of each y-axis row (−Xpix/2), move        across the corresponding row of Mk(x,y) toward the right end of        the row until an edge is detected by a transition from 0 to a        non-zero value (e.g., 1). Replace the first element having a        non-zero value with a value of 0.85 in array Mk(x,y). Then for        the next 7 cm to 12 cm of the row, move to the right by one        pixel or super pixel at a time, and replace the corresponding        Mk(x,y) with a value that is in accordance with one of the        dashed lines of FIG. 17 if the Mk(x,y) value is greater than the        replaced Mk(x,y) value of the previously processed pixel or        super pixel. If, during this processing, the current Mk(x,y)        value is less than the Mk(x,y) value of the previously processed        pixel, then stop this action as the right edge has been reached.    -   Action #3. From the right edge of each y-axis row (+Xpix/2),        move across the corresponding row of Mk(x,y) toward the left end        of the row until an edge is detected by a transition from 0 to a        non-zero value (e.g., 1). Replace the first element having a        non-zero value with a value of 0.85 in array Mk(x,y). Then for        the next 7 cm to 12 cm of the row, move to the left by one pixel        or super pixel at a time, and replace the corresponding Mk(x,y)        with a value that is in accordance with one of the dashed lines        of FIG. 17 if the Mk(x,y) value is greater than the replaced        Mk(x,y) value of the previously-processed pixel or super pixel.        If, during this processing, the current Mk(x,y) value is less        than the Mk(x,y) value of the previously processed pixel, then        stop this action as the processed left edge has been reached.        The sequence of actions #2 and #3 may be exchanged, with action        #3 being performed before action #2.

A second exemplary method of generating the kernel modification functionMk(x,y) according to the dashed straight line is as follows.

-   -   Action #1. For each pixel or super pixel, set the value of the        array element Mk(x,y) corresponding to a pixel or super pixel to        1 if the value of the measured radiation profile I_(I)(x,y) at        the pixel or super pixel is below a threshold value I_(TH),        otherwise set the value to a value of (1-2Δ) where Δ is the        relative amount of the edge effect (˜0.15 for the curve shown in        FIG. 17). The threshold value I_(TH) may be at or slightly below        the value of I₀. Values of I_(I)(x,y) above I_(TH) indicate the        absence of the object, while values below I_(TH) indicate the        presence of the object.    -   Action #2. Convolve the Mk(x,y) values of each row with a        rectangle function having an extent from −7 cm to +7 cm and a        value of 1 within that extent, and a value of zero outside of        that extent. (The rectangle function is also called a uniform        horizontal window.) Before the convolution, a padding extension        may be added to each side of each row of Mk(x,y) values; each        padding extension has a length equal to the extent of the        rectangle function (e.g., 14 cm), and has uniform Mk(*) values        of (1-2Δ).    -   Action #3. Remove the padding extensions (if used), and set the        values that are below (1-Δ) to zero.        The second exemplary method may be readily adapted to use, in        action #2, a two-dimensional convolution of Mk(*) and a        two-dimensional rectangle function over the two-dimensional        domain of Mk(*) (with optional padding extension). Such a        convolution may be done with Fourier transforms.

When compensating for edge effects in the content of the iterativesolutions [13]-[15], [21]-[23], [37]-[39], and [45]-[47], the Mk(x,y)may be generated once per projection and used for all the iterations ofthe solution since the structure of the array need only depend upon themeasured data I_(I)(x,y) for the projection.

Another method of compensating for edge effects, and more generally thechanges in secondary scattering events, is to modify the kernel constant“A” according to the formA′(x _(m) ,y _(n))=A·{1+δ·[τ_(AVG)(x _(m) ,y _(n))−τ(x _(m) ,y_(n))]},  [48]where δ is a constant coefficient, point (x_(m),y_(n)) is the locationon the pixel array where the pencil beam terminates, τ(x_(m),y_(n)) isthe object thickness seen by the pencil beam, and τ_(AVG)(x_(m),y_(n))is the average thickness of the object seen by a small cone of otherpencil beams around the pencil beam. A′(x_(m), y_(n)) then replaces theconstant A in the equations as a matrix comprising spatially variantscaling. Equation [48] is a more generalized way of accounting foreffects of thickness changes for the edge effects. τ(x_(m),y_(n)) may beprovided by the previously described thickness functionT_(I)(I₀,I_(bp)). τ_(AVG)(x_(m),y_(n)) may be generated over a smallgroup of pixels around point (x_(m),y_(n)) as follows:

$\begin{matrix}{{\tau_{AVG}\left( {x_{m},y_{n}} \right)} = {{\frac{1}{4 \cdot R_{X} \cdot R_{Y}} \cdot {\sum\limits_{y = {y_{n} - R_{Y}}}^{y = {y_{n} + R_{Y}}}{\sum\limits_{x = {x_{m} - R_{X}}}^{x = {x_{m} + R_{X}}}{\tau\left( {x,y} \right)}}}} \approx {\frac{1}{4 \cdot R_{X} \cdot R_{Y}} \cdot {\sum\limits_{y = {y_{n} - R_{Y}}}^{y = {y_{n} + R_{Y}}}{\sum\limits_{x = {x_{m} - R_{X}}}^{x = {x_{m} + R_{X}}}{{T_{I}\left( {I_{0},{I_{bp}\left( {x,y} \right)}} \right)}.}}}}}} & \lbrack 49\rbrack\end{matrix}$Using the division property of logarithms, T_(I)(*) can be written asT_(J)(I₀,I_(bp))=ln {I₀(x_(m),y_(n))/I_(bp)(x_(m),y_(n))}/μ=(1/μ)·[ln{I₀(x_(m),y_(n))}−ln {I_(bp)(x_(m),y_(n))}]. Generally, I₀(x_(m),y_(n))will be uniform in the small area around a point of and edge, andaverages to the same value under the approximation of equation [49], andwill cancel with a similar term in equation [48] for T_(I)(I₀,I_(bp)).Equation [48] can then be approximated as:

$\begin{matrix}{{{A^{\prime}\left( {x_{m},y_{n}} \right)} \approx {A \cdot \left\{ {1 + {\delta \cdot \left\lbrack {{{AVG}\left( {{{- \left( {1/\mu} \right)} \cdot \ln}\left\{ {I_{bp}\left( {x_{m},y_{n}} \right)} \right\}} \right)} + {{\left( {1/\mu} \right) \cdot \ln}\left\{ {I_{bp}\left( {x_{m},y_{n}} \right)} \right\}}} \right\rbrack}} \right\}}},{= {A \cdot \left\{ {1 + {\left( {\delta/\mu} \right) \cdot \left\lbrack {{\ln\left\{ {I_{bp}\left( {x_{m},y_{n}} \right)} \right\}} - {{AVG}\left( {\ln\left\{ {I_{bp}\left( {x_{m},y_{n}} \right)} \right\}} \right)}} \right\rbrack}} \right\}}}} & \lbrack 50\rbrack\end{matrix}$where AVG(*) is an averaging function. The approximation of equation[50] is simpler, but comprises the essential part of the approximationof equation [49] (both involve generating a difference between thelogarithm of I_(bp)(x,y) at an edge location and the average of thelogarithm of I_(bp)(x,y) around the edge location). Since both δ and μare constants, they may be replaced by a single constant in equation[50].

Scatter-Anchoring.

In some cases, there may be systematic errors in the scatter estimategenerated by the above inventions. For example, if the object isoff-center, then, for those projections where the object is closer tothe detector than is expected, the scatter model will underestimateobject scatter, whereas, for projections situated 180 degrees away, theobject scatter model will overestimate object scatter. Other sources ofsystematic error may include the presence of material other than thematerial (e.g., water) upon which the object scatter kernels are based.

A first approach of addressing such systematic errors is to perform afirst tomographic reconstruction of the object, and then adjust thekernels during a second subsequent tomographic reconstruction. Anotherapproach is to measure scatter at one or more edges of the detector anduse the measurements to scale the scatter estimate derived from thekernel model to bring the kernel model into agreement with themeasurements. This approach is referred to as “anchoring,” and isdescribed in greater detail below.

One approach of measuring the scatter is to shade one or more edges ofthe detector with collimator blades so that the direct radiation fromradiation source 110 is effectively blocked in one or more bands at oneor more corresponding edges of imaging device 120. This causes thenon-scattered radiation I_(o0) from the object to be substantially zerowithin the bands. One or both sets of fan blades 130-140 shown in FIGS.2-4 may be used to provide the shading that creates these bands. Thebands may be made sufficiently wide to prevent leakage of thenon-scattered radiation I_(o0) at a sub-band at the very edge, theleakage being caused by the beam penumbra of the source (source 110 isnot exact a point source, and has a finite width that causes thepenumbra). FIG. 18 illustrates a configuration where fan blades 130-140are positioned to provide shaded bands around all sides of imagingdevice 120, each band including a penumbra region and a sub-band withonly scattered radiation (no primary signal from the object). Thedetector values of the pixels in these sub-bands effectively measure thescattered radiation.

The estimate of the scattered radiation generated using the kernels,such as by equations [14C],[22D],[38D], and [46D], may be compared tothe detector values in the sub-bands for each radiographic projection.(If desired, the scatter in the sub-bands may be corrected for anyscattering caused by the detector housing; such correction has a minimaleffect since the object scattering is slowly varying.) If there is asignificant difference, a scaling factor may be introduced intoequations [13]-[15], [21]-[23], [37]-[39], and [45]-[47] to adjust thekernel model and bring it into agreement with measurements for theprojection. The scaling factor may be a global constant that ismultiplied with the scatter value generated for each super pixel or realpixel, or a global constant that is added with the scatter valuegenerated for each super pixel or real pixel (or a combination of bothapproaches). The global constant may be generated from a least squaresfitting method that minimizes the difference between the measuredscatter radiation of the sub-bands and the scattered radiation generatedby the kernel model, as adjusted by the global constant, at one or morelocations in the sub-bands. Least squares fitting methods are well knownto the mathematics art, and one of ordinary skill in the art can makeand use these inventions of the present application without undueexperimentation in view of this specification, and without a detailedexplanation of least squares fitting methods. While FIG. 18 illustratesthis invention with the use of measured data from all four edges of thepixel array, one may use the measured data from just one, two or, threeedges. In addition, a specially configured collimator may be used inplace of the fan blades, where the special collimator shades one or moresmall regions along one or more of the edges, rather than whole edges,and the scatter anchoring is performed using one or more of these smallregions. Such regions may be at the corners of the pixel array, and/orthe midpoints of the edges, and/or points between.

In addition, more elaborate fitting surfaces may be used. For example,the scaling factor may be allowed to vary linearly along each edge (orcollinear set of small regions), and a least squares fitting method maybe used to find a slope and offset for each edge. As another approach, asecond or higher order polynomial may be used to fit the scaling factoralong each edge (or collinear set of small regions). Then, the scalingfactor may be interpolated to the interior of the array using anyconventional interpolation method. While this method has beenillustrated using all four edges, it may be implemented using one, two,or three edges, or with one or more small regions provided by thespecially configured collimator. As another method, the scaling factorsmay be estimated for several selected areas along one or more of theedges, and a mathematical plane may be fitted to these scaling factorsby a least squares fitting method. The equation of the mathematicalplane may then be used to generate the scaling factors for all of thepixels and super pixels of the array.

A disadvantage of completely shading the edges of the detector to getthe sub-bands of pure scattered radiation is that the imagingfield-of-view is reduced. Another approach, which addresses thisdisadvantage, accounts for the penumbra effect and enables less shadingto be used, such as shown by FIG. 19, to generate estimates of thescaling factors. We first examine the effect of the penumbra for thecase where no object is present. In FIG. 20, the radiation intensity isplotted as a distance “r” along a row or column of the pixel array, nearthe array edge. The penumbra effect starts at r=0, and ends at r=Wp,where Wp is the width of the penumbra. In the unshaded region to theleft, the radiation intensity I_(I)(r) incident upon the detector coveris at a value of I₀, as expected. In the penumbra region, I_(I)(r)decreases linearly with distance, which may be written asI_(I)(r)=I₀·(1−a·r), where “a” is a slope constant. We can assume that,when the object is present, its thickness will be substantially constantat the edge of the column or row. Under this assumption, the primaryradiation from the object (e.g., I_(o0))) will decrease linearly withdistance r in the penumbra region with the same slope “a”. FIG. 21 plotsthe ideal radiation intensity in the penumbra region when the object ispresent. It has two contributions: the primary radiation I_(o0) from theobject, which is assumed to decrease as P₀·(1−a·r), and the scattercontribution, which is assumed to be a constant SC. As shown in thefigure, the radiation intensity I_(I)(r) incident at the detectorhousing, which can be estimated from measurements as described above,can be written as I_(I)(r)=I_(o0)(r)+SC=−a·P₀·r+(SC+P₀) for regions of rbetween 0 and Wp. FIG. 22 shows a plot of the measured intensity at theedge of a penumbra region for a column or row or pixels. The measuredradiation intensity only extends through a portion of the PenumbraRegion. A linear line I=m·r+b may be fitted to the sloping intensity inthe portion using a least squares fitting method. From FIG. 21, we seethat the parameter “m” is equal to m=−a·P₀ and the parameter “b” isequation to (SC+P₀). From this, the scattered radiation at the edge canbe found as SC=b+m/a. This approach works well when done using superpixels, where the measured data is averaged to provide representativedata values for the super pixels, as described above. However, becausethe slope may be relatively steep, it may be desired to reduce the sizeof the super pixels in the direction of r, as compared to the size ofthe super pixels in the direction r for measuring object scatter, inorder to realize a better fit of the slope. For each edge, the slopemethod can be applied either once to generate one SC value, or severaltimes for several rows (or columns), up to as many as the number ofpixels along that edge, to generate SC for the several rows (orcolumns). (Here, a row or column may be of the super pixel array or thereal array.) A globally constant or variant scaling factor can then begenerated as previously described above.

Using the above approach, the scattered radiation values SC can beestimated at the penumbra edges using a much smaller amount of shading.These SC values may then be interpolated over the entire array in thevarious ways discussed above. Also, the specially configured collimatormay be used in place of the fan blades to shade one or more smallregions along one or more of the edges, rather than whole edges, and thescatter anchoring may be performed using one or more of these smallregions. Such regions may be at the corners of the pixel array, and/orthe midpoints of the edges, and/or points between.

Additional Projection Correction Methods.

In addition to the above methods of generating corrected projections ofthe object (I_(o0)(x,y)), a corrected radiographic projection may begenerated as a truncated difference between the uncorrected projectionI_(I)(x,y) and the final scatter component I^(K) _(os)(x,y). Thetruncated difference may comprise generating the ratio of the scattercomponent I^(K) _(os)(x,y) to the uncorrected projection I_(I)(x,y),processing the ratio by filtering its values over the spatial domainand/or limiting its values of the ratio to a range (such 0 to 0.8), andthen multiplying I_(I)(x,y) by one minus the processed ratio. This maybe expressed as:I _(o0)(x,y)=I _(I)(x,y)·{1−TRUNC(I ^(K) _(os)(x,y)/I _(I)(x,y)}.  [51]

Results of Scatter Estimations.

As indicated above, the estimates of scattered radiation in radiographicprojections may be used to correct the projections, such as bysubtracting the scatter estimates from the pixel values of theradiographic projections. The corrected projections may then be used in3-D tomographic reconstructions of the object. We now show some resultsof the use of such corrections, with the scatter estimate beinggenerated using the above-described multiple asymmetric kernels. FIG. 23shows a cross section of a pelvic phantom from a tomographicreconstruction before correction, and FIG. 24 shows the same crosssection generated from projections that have been corrected. Variousscatter artifacts can be seen in FIG. 23, such as the circular-shapedhalos. These artifacts are significantly reduced in the corrected crosssection of FIG. 24. The range of the Hounsfield Unit (HU) uncertainty isalso significantly reduced from approximately zero to −260 HU to +30 to−50 HU. FIGS. 25A-25D plot the uncorrected pixel values along a crosssection of four different radiographic projects taken of an object atfour different angles, 90-degrees apart from one another. Also shown inFIGS. 25A-25D is a measured value of scattered radiation and theestimated values of scattered radiation provided by an exemplaryembodiment of the asymmetric multi-kernel inventions of the presentapplication. As can be seen in the figures, the estimated scattermatches the measured scattered very well.

System and Computer Program Product Embodiments.

Each of the above-described methods may be implemented by computerprogram products that direct a computer system to perform the actions ofthe methods. Each such computer program product may comprise sets ofinstructions embodied on a tangible computer-readable medium that directthe processor of a computer system to perform the corresponding actions.Examples of such computer-readable mediums are the instruction memoryshown for controller 160 in FIGS. 2A and 2B. The instructions may be inexecutable computer code (such as C, C++), human-readable code (such asMatLab Code), and the like. Other types of tangible computer-readablemedia include floppy disks, removable hard disks, optical storage mediasuch as CD-ROMS, DVDs and bar codes, semiconductor memories such asflash memories, read-only-memories (ROMS), battery-backed volatilememories, networked storage devices, and the like. Given theabove-detailed description of the various method embodiments of theinventions of the present application, it is within the skill of one ofordinary skill in the tomography art to implement each of the methodembodiments disclosed herein in a computer program product without undueexperimentation. Such computer program product may be run on processor160 shown in FIGS. 2A and 2B, or on separate processors that are notcoupled to Cone-beam Computer Tomography Systems.

Exemplary system inventions of the present application may compriseradiation source 110, imaging device 120, and controller 160, incombination with various computer program product inventions and/ormethods of the present application.

Any recitation of “a”, “an”, and “the” is intended to mean one or moreunless specifically indicated to the contrary.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, it being recognized that variousmodifications are possible within the scope of the invention claimed.

Moreover, one or more features of one or more embodiments of theinvention may be combined with one or more features of other embodimentsof the invention without departing from the scope of the invention.

While the present invention has been particularly described with respectto the illustrated embodiments, it will be appreciated that variousalterations, modifications, adaptations, and equivalent arrangements maybe made based on the present disclosure, and are intended to be withinthe scope of the invention and the appended claim.

What is claimed is:
 1. A method of estimating scattered radiation in aradiographic projection of an object, the radiographic projectiongenerated by an imaging device irradiated by a radiation source spacedtherefrom to provide a space for the object, the imaging devicemeasuring incident radiation at a plurality of pixels at correspondinglocations, the radiographic projection comprising a plurality ofradiation values corresponding to a plurality of pixel locations of theimaging device, each radiation value comprising a primary radiationamount representative of the radiation reaching the corresponding pixelalong a direct path from the radiation source and a scattered radiationamount representative of other radiation reaching the correspondingpixel, the method comprising: obtaining an estimate of a radiationamount associated with a first location of the radiographic projection,the radiation amount comprising one of a radiation amount emitted by theradiation source or the primary radiation amount at the first location;generating an estimate of scattered radiation at a plurality oflocations of the radiographic projection using the estimate of theradiation amount at the first location and a kernel that generates avalue representative of scattered radiation for a location of theradiographic projection in relation to the distance between thatlocation and the first location, the kernel having a form that isasymmetric about the location of the first pixel location, wherein thetwo-dimensional array of the imaging device comprises a flat surface,and wherein generating an estimate of scattered radiation further uses askew correction factor to at least partially correct for skewing of datavalues due to the projection of the scattered radiation onto a flatsurface, wherein the skew correction factor for a second location of theplurality of locations includes: a first term including a first distancealong a first dimension x between the first location and the secondlocation, the first distance multiplied by a first angle term includinga first angle of a direct path from the radiation source to the firstlocation, the first angle resulting from a projection of the direct pathonto a first plane including the first dimension x; a second termincluding a second distance along a second dimension y between the firstlocation and the second location, the second distance multiplied by asecond angle term including a second angle of the direct path from theradiation source to the second location, the second angle resulting froma projection of the direct path onto a second plane including the seconddimension y; and storing the estimate of the scattered radiation in acomputer-readable medium.
 2. The method of claim 1 further comprisingobtaining a plurality of estimates of the object's thickness along aplurality of direct paths from the radiation source to a correspondingplurality of locations of the imaging device and the radiographicprojection; and wherein the form of the kernel varies about the firstlocation of the radiographic projection in relation to the estimates ofthe object thickness about the first location.
 3. The method of claim 1wherein the value of the kernel at a location is related to a differencebetween the estimate of object thickness at the location and theestimate of object thickness at the first location.
 4. The method ofclaim 1 wherein the form of the kernel varies about the first locationof the radiographic projection in relation to estimates of the primaryradiation amount about the first location.
 5. The method of claim 4wherein the value of the kernel at a location is related to a differencebetween a logarithm of the primary radiation amount at the location anda logarithm of the primary radiation amount at the first location. 6.The method of claim 1 wherein the kernel comprises an amplitude factorand an asymmetric form factor that is a function of at least one a setof estimates of the object thickness, and wherein the kernel furthercomprises a symmetric form factor, and wherein the asymmetric formfactor comprises a component that is mathematically equivalent to{1+ƒ₁(τ_(xy), τ_(hp))}, where τ_(xy) is an estimate of the object'sthickness along a direct path from the radiation source to a location ofthe radiographic projection for which scattered radiation is beingestimated or at a location between there and the first location, andτ_(hp) is an estimate of the object's thickness along a direct path fromthe radiation source to the first location of the radiographicprojection, and where ƒ₁(*) is a function of at least τ_(xy) and τ_(bp).7. The method of claim 1 wherein the kernel comprises an amplitudefactor and an asymmetric form factor that is a function of a set ofestimates of the primary radiation in the radiographic projection, andwherein the kernel further comprises a symmetric form factor, andwherein the asymmetric form factor comprises a component that ismathematically equivalent to {1+ƒ₂(I_(bp,LI), I_(bp))}, I_(bp,LI) is anestimate of the primary radiation amount at the first location, andI_(bp) is an estimate of the primary radiation amount at a location ofthe radiographic projection for which scattered radiation is beingestimated or at a location between there and the first location, andwhere ƒ₂(*) is a function of at least I_(bp,LI) and I_(bp).
 8. Themethod of claim 1, wherein the first distance varies from positive tonegative depending on whether the second location has a smaller orlarger value for the x-coordinate.
 9. The method of claim 1, wherein theskew correction factor comprises a third term that includes a thirdangle, the third angle being between the first path and a projectionaxis from the radiation source to a center of the flat surface.
 10. Amethod of estimating scattered radiation in a radiographic projection ofan object, the radiographic projection generated by an imaging deviceirradiated by a radiation source spaced therefrom to provide a space forthe object, the imaging device measuring incident radiation at aplurality of pixels at corresponding locations, the radiographicprojection comprising a plurality of radiation values corresponding to aplurality of pixel locations of the imaging device, each radiation valuecomprising a primary radiation amount representative of the radiationreaching the corresponding pixel along a direct path from the radiationsource and a scattered radiation amount representative of otherradiation reaching the corresponding pixel, the method comprising:obtaining an estimate of a radiation amount associated with a firstlocation of the radiographic projection, the radiation amount comprisingone of a radiation amount emitted by the radiation source or the primaryradiation amount at the first location; generating an estimate ofscattered radiation at a plurality of locations of the radiographicprojection using the estimate of the radiation amount at the firstlocation and a kernel that generates a value representative of scatteredradiation for a location of the radiographic projection in relation tothe distance between that location and the first location, the kernelhaving a form that is asymmetric about the location of the first pixellocation, wherein the kernel comprises an amplitude factor and anasymmetric form factor that is a function of at least one a set ofestimates of the object thickness and a set of estimates of the primaryradiation in the radiographic projection, wherein the asymmetric formfactor for a second location of the plurality of locations includes: afirst term corresponding to a first thickness of the object along thedirect path to the first location, and a second term corresponding to asecond thickness of the object along a scattered path to the secondlocation, the scattered path being from a point EP that the direct pathreaches the object to the second location; and storing the estimate ofthe scattered radiation in a computer-readable medium.
 11. The method ofclaim 10 further comprising obtaining a plurality of estimates of theobject's thickness along a plurality of direct paths from the radiationsource to a corresponding plurality of locations of the imaging deviceand the radiographic projection for areas along and around the at leastone edge, and wherein adjusting the values of the amplitude factor inareas around the at least one edge comprises adjusting at least one suchvalue for a location in relation to the difference between the averageobject thickness around the location and the object thickness at thelocation.
 12. The method of claim 10 wherein the radiation amounts forthe locations of the radiographic projection comprise the primaryradiation amounts I_(bp)(x,y) at the locations, and wherein adjustingthe values of the amplitude factor in areas around the at least one edgecomprises adjusting at least one such value for a location in relationto the difference between a logarithm of I_(bp)(x,y) at the location andthe average of the logarithm of I_(bp)(x,y) around the location.
 13. Themethod of claim 10, wherein the amplitude factor varies with location onthe radiographic projection, wherein generating the estimate ofscattered radiation further comprises finding at least one edge of theobject in the radiographic projection and adjusting the values of theamplitude factor in areas around the at least one edge.
 14. The methodof claim 10, further comprising: estimating a thickness of the object ata plurality of points of the radiographic projection, wherein the secondthickness is determined by: determining a third location whose X,Ycoordinates are offset from the first location by an amount proportionalto a difference between the second location and the first location. 15.The method of claim 14, wherein the third location is the secondlocation.
 16. The method of claim 14, wherein the kernel has a form thatis asymmetric about the corresponding location.
 17. The method of claim14, wherein generating a reference estimate includes: identifying adecrease in the primary radiation amount at corresponding locations inthe at least one partially shared area; and determining the referenceestimate based on the decrease.
 18. The method of claim 17, whereindetermining the reference estimate based on the decrease includes:fitting the decrease in primary radiation amounts to a linear functionhaving a slope m; and computing the reference estimate using the primaryradiation amount at a first corresponding location, the slope m, and aknown slope a corresponding to a decrease that is independent of theprimary radiation amount at the first corresponding location.
 19. Amethod of estimating scattered radiation in a radiographic projection ofan object, the radiographic projection generated by an imaging deviceirradiated by a radiation source spaced therefrom to provide a space forthe object, the imaging device measuring incident radiation at aplurality of pixels at corresponding locations, the radiographicprojection comprising a plurality of radiation values corresponding to aplurality of pixel locations of the imaging device, each radiation valuecomprising a primary radiation amount representative of the radiationreaching the corresponding pixel along a direct path from the radiationsource and a scattered radiation amount representative of otherradiation reaching the corresponding pixel, the method comprising:obtaining a plurality of estimates of radiation amounts at a pluralityof corresponding locations of the radiographic projection, eachradiation amount comprising one of a radiation amount emitted by theradiation source or the primary radiation amount at the correspondinglocation, wherein the radiographic projection comprises at least onearea that was partially shaded from source radiation by a second object,the shading reducing the primary radiation amount in radiation values inthe at least one area but not substantially reducing the scatteredradiation amount in the radiation values, the at least one area beingbetween a first region that was less shaded and a second region that ismore shaded than the at least one area; generating an estimate ofscattered radiation at a plurality of locations of the radiographicprojection using the obtained estimates of the radiation amount and akernel that generates a value representative of scattered radiation fora location of the radiographic projection in relation to the distancebetween that location and a corresponding location; generating areference estimate of at least one scattered radiation amount in the atleast one partially shaded area; adjusting the estimate of scatteredradiation using the reference estimate; and storing the estimate of thescattered radiation in a computer-readable medium.
 20. The method ofclaim 19 further comprising obtaining a plurality of estimates of theobject's thickness along a plurality of direct paths from the radiationsource to a corresponding plurality of locations of the imaging deviceand the radiographic projection; and wherein the form of the kernelvaries about the first location of the radiographic projection inrelation to the estimates of the object thickness about the firstlocation.
 21. The method of claim 19 wherein the form of the kernelvaries about the first location of the radiographic projection inrelation to estimates of the primary radiation amount about the firstlocation.
 22. A method of estimating scattered radiation in aradiographic projection of an object, the radiographic projectiongenerated by an imaging device irradiated by a radiation source spacedtherefrom to provide a space for the object, the imaging devicemeasuring incident radiation at a plurality of pixels at correspondinglocations, the radiographic projection comprising a plurality ofradiation values corresponding to a plurality of pixel locations of theimaging device, each radiation value comprising a primary radiationamount representative of the radiation reaching the corresponding pixelalong a direct path from the radiation source and a scattered radiationamount representative of other radiation reaching the correspondingpixel, the method comprising: correcting the radiation values of theradiographic projection for scattered radiation caused by a housing ofan imaging device; obtaining an estimate of a radiation amountassociated with a first location of the radiographic projection, theradiation amount comprising one of a radiation amount emitted by theradiation source or the primary radiation amount at the first location;generating an estimate of scattered radiation at a plurality oflocations of the radiographic projection using the estimate of theradiation amount at the first location and a kernel that generates avalue representative of scattered radiation for a location of theradiographic projection in relation to the distance between thatlocation and the first location, the kernel having a form that isasymmetric about the location of the first pixel location; and storingthe estimate of the scattered radiation in a computer-readable medium.23. The method of claim 22 further comprising obtaining a plurality ofestimates of the object's thickness along a plurality of direct pathsfrom the radiation source to a corresponding plurality of locations ofthe imaging device and the radiographic projection; and wherein the formof the kernel varies about the first location of the radiographicprojection in relation to the estimates of the object thickness aboutthe first location.
 24. The method of claim 22 wherein the value of thekernel at a location is related to a difference between the estimate ofobject thickness at the location and the estimate of object thickness atthe first location.
 25. The method of claim 22 wherein the form of thekernel varies about the first location of the radiographic projection inrelation to estimates of the primary radiation amount about the firstlocation.
 26. The method of claim 25 wherein the value of the kernel ata location is related to a difference between a logarithm of the primaryradiation amount at the location and a logarithm of the primaryradiation amount at the first location.
 27. A method of estimatingscattered radiation in a radiographic projection of an object, theradiographic projection generated by an imaging device irradiated by aradiation source spaced therefrom to provide a space for the object, theimaging device measuring incident radiation at a plurality of pixels atcorresponding locations, the radiographic projection comprising aplurality of radiation values corresponding to a plurality of pixellocations of the imaging device, each radiation value comprising aprimary radiation amount representative of the radiation reaching thecorresponding pixel along a direct path from the radiation source and ascattered radiation amount representative of other radiation reachingthe corresponding pixel, the method comprising: obtaining estimates ofthe primary radiation amounts at a plurality of corresponding locationsof the radiographic projection; generating an estimate of scatteredradiation at a plurality of locations of the radiographic projectionbased upon the estimates of the primary radiation amounts at theplurality of corresponding locations of the radiographic projection, anamplitude factor S_(A)(*), and a symmetric form factor S_(FF)(x,y),wherein generating the estimate of scattered radiation comprises:generating a first quantity representative of a convolution of thesymmetric form factor S_(FF)(x,y) and estimates of the primary radiationamounts scaled by the amplitude factor S_(A)(*), multiplying the firstquantity by a first function of the estimates of radiation amounts, andgenerating a second quantity representative of the symmetric form factorS_(FF)(x,y) convolved with estimates of the primary radiation amountsthat has been scaled by the amplitude factor S_(A)(*) and multiplied bya second function of the estimates of the primary radiation amounts; andstoring the estimate of the scattered radiation in a computer-readablemedium.
 28. The method of claim 27, wherein the first function comprisesunity plus a scaled representation of the plurality of logarithms of theestimates of the primary radiation amounts, and wherein the secondfunction comprises a scaled representation of the plurality oflogarithms of the estimates of the primary radiation amounts.
 29. Themethod claim 27, wherein the first and second quantities are generatedusing at least one of the following: a Fourier transform of thesymmetric form factor S_(FF)(x,y), a Fourier transform of the set ofestimates of the primary radiation amounts scaled by the amplitudefactor S_(A)(*), a Fourier transform of a set of estimates of theprimary radiation amounts that has been scaled by the amplitude factorS_(A)(*) and multiplied by the second function of the estimates of theprimary radiation amounts.